oracle database 11g using ddl views sequences indexes and synonyms

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Creating Tables and Data Types

Learning objective

After completing this topic, you should be able to recognize the steps for creating, defining, and naming a table, and for specifying data types for its columns.

1. Creating, defining, and naming a table

Disclaimer

Although certain aspects of Oracle Database 11g are case and spacing insensitive, a common coding convention has been used throughout all aspects of this course.

This convention uses lowercase characters for schema, role, user, and constraint names, and for permissions, synonyms, and table names (with the exception of the DUAL table). Lowercase characters are also used for column names and user-defined procedure, function, and variable names shown in code.

Uppercase characters are used for Oracle keywords and functions, for view, table, schema, and column names shown in text, for column aliases that are not shown in quotes, for packages, and for data dictionary views.

The spacing convention requires one space after a comma and one space before and after operators that are not Oracle-specific, such as +, -, /, and <. There should be no space between an Oracle-specific keyword or operator and an opening bracket, a closing bracket and a comma, between the last part of a statement and the closing semicolon, or before a statement.

String literals in single quotes are an exception to all convention rules provided. Please use this convention for all interactive parts of this course.------------------------------------------------------------------------------------------------------------

An Oracle database can contain multiple data structures. Each of these structures should be outlined in the database design so that they can be created during the build stage of database development.

These are some of the objects in an Oracle database:

table

view

sequence

index

synonym

table

A table is the basic unit of data storage. It is composed of rows.

view

A view logically represents subsets of data from one or more tables.

sequence

A sequence generates numeric values.

index

An index improves the performance of some queries.

synonym

A synonym gives an alternative name to an object.

Oracle table structures have some characteristic features. These include

You can create tables at any time, even when users are using the database.

You do not need to specify the size of a table. The size is ultimately defined by the amount of space allocated to the database as a whole. It is important, however, to estimate how much space a table will use over time.

You can modify the table structure online.

You can name Oracle database tables and columns according to these standard rules for naming an Oracle database object:

table names and column names must begin with a letter and be 1–30 characters long

names must contain only the characters A–Z, a–z, 0–9, _ (underscore), $, and #. The $ and # characters are legal characters, but their use is discouraged

names must not duplicate the name of another object owned by the same Oracle server user

names must not be Oracle server-reserved words

You may also use quoted identifiers to represent the name of an object. A quoted identifier begins and ends with double quotation marks (").

If you name a schema object using a quoted identifier, then you must use the double quotation marks whenever you refer to that object. Quoted identifiers can be reserved words, although this is not recommended.

An important naming guideline for tables is to use descriptive names for tables and other database objects.

Note

Names are not case-sensitive. For example, EMPLOYEES is treated to be the same as eMPloyees or eMpLOYEES. However, quoted identifiers are case-sensitive.

You can create tables to store data using

the CREATE TABLE statement

the AS Subquery clause

The CREATE TABLE statement is one of the DDL statements, which are a subset to the SQL statements used for creating, modifying, or removing Oracle database structures.

These statements have an immediate effect on the database and they also record information in the data dictionary.

To create a table, a user must have the CREATE TABLE privilege and a storage area in which to create objects. The database administrator (DBA) uses data control language (DCL) statements to grant privileges to users.

This is the syntax for creating tables by using the CREATE TABLE statement:

CREATE TABLE [schema.]table(column datatype [DEFAULT expr][, ...]);

The various elements of the syntax for creating tables by using the CREATE TABLE statement are

schema

table

DEFAULT expr

column

datatype

schema

The schema element in the syntax is the same as the owner's name.

table

The table element in the syntax is the name of the table.

DEFAULT expr

The DEFAULT expr element specifies a default value if a value is omitted in the INSERT statement.

column

The column element is the name of the column.

datatype

The datatype element is the column's data type and length.

Using the AS subquery clause with the CREATE TABLE statement creates the table and inserts rows returned from the subquery.

In the syntax for the AS subquery clause

table is the name of the table

column is the name of the column, default value, and integrity constraint

subquery is the SELECT statement that defines the set of rows to be inserted into the new table

CREATE TABLE table[(column, column...)]AS subquery;

Here are some guidelines for creating tables using the AS subquery clause:

the table is created with the specified column names, and the rows retrieved by the SELECT statement are inserted into the table

the column definition can contain only the column name and default value

if column specifications are given, the number of columns must equal the number of columns in the subquery SELECT list

Here are some more guidelines for creating tables using the AS subquery clause:

If no column specifications are given, the column names of the table are the same as the column names in the subquery.

The column data type definitions and the NOT NULL constraint are passed to the new table. Note that only the explicit NOT NULL constraint will be inherited. The PRIMARY KEY column will not pass the NOT NULL feature to the new column. Any other constraint rules are not passed to the new table. However, you can add constraints in the column definition.

This sample code creates a table named DEPT80, which contains details of all the employees working in department 80.

Notice that the data for the DEPT80 table comes from the EMPLOYEES table.

CREATE TABLE dept80AS SELECT employee_id, last_name, salary*12 ANNSAL, hire_date FROM employees WHERE department_id = 80;

--------------------------------CREATE TABLE succeeded

You can verify the existence of a database table and check the column definitions by using the DESCRIBE command.

DESCRIBE dept80

It is important to provide a column alias when selecting an expression in a subquery.

The expression SALARY*12 is given the alias ANNSAL. Without the alias, the error in this sample code is generated.

CREATE TABLE dept 80AS SELECT employee_id, last_name, salary*12hire_date FROM employees WHERE department_id = 80

A schema is a collection of logical structures of data or schema objects. It is owned by a database user and has the same name as the user. Each user owns a single schema.

Schema objects can be created and manipulated with SQL and include tables, views, synonyms, sequences, stored procedures, indexes, clusters, and database links.

Here's how you can reference another user's tables. If a table does not belong to the user, the owner's name must be prefixed to the table.

For example, assume that there are schemas named USERA and USERB, and both have an EMPLOYEES table. If USERA wants to access the EMPLOYEES table that belongs to USERB, USERA must prefix the table name with the schema name.

SELECT *FROM userb.employees;

If USERB wants to access the EMPLOYEES table that is owned by USERA, USERB must prefix the table name with the schema name.

SELECT *FROM usera.employees;

When you define a table, you can specify that a column should be given a default value by using the DEFAULT option. This option prevents null values from entering the columns, when a row is inserted without a value for the column.

The default value can be a literal, an expression, or a SQL function, such as SYSDATE or

USER, but the value cannot be the name of another column or a pseudocolumn, such as NEXTVAL or CURRVAL. The default expression must match the data type of the column.

CREATE TABLE hire_dates (id NUMBER(8), hire_date DATE DEFAULT SYSDATE);---------------------------------CREATE TABLE succeeded.

This sample code inserts the null value rather than the default value.

INSERT INTO hire_dates values(45, NULL);

This sample code inserts SYSDATE for the HIRE_DATE column.

INSERT INTO hire_dates(id) values(35);

Note

In SQL Developer, you click the Run Script icon or press F5 to run the DDL statements. The feedback messages will be shown on the Script Output tabbed page.

This sample code creates the DEPT table with four columns: DEPTNO, DNAME, LOC, and CREATE_DATE.

The CREATE_DATE column has a default value. If a value is not provided for an INSERT statement, the system date is automatically inserted.

CREATE TABLE dept (deptno NUMBER(2), dname VARCHAR2(14),loc VARCHAR2(13),create_date DATE DEFAULT SYSDATE);----------------------------CREATE TABLE succeeded.

To confirm that the table was created, you run the DESCRIBE command.

Because creating a table is a DDL statement, an automatic commit takes place when the CREATE TABLE statement is executed.

DESCRIBE dept

Question

Which Oracle Database 11g structure generates numeric values?

Options:

1. Sequence

2. Synonym

3. Table

4. View

Answer

A sequence generates numeric values.

Option 1 is correct. Sequences are database structures used to generate numeric values for numeric columns of a database's tables.

Option 2 is incorrect. Synonyms give an alternative name to an object. Synonyms do not generate numeric values.

Option 3 is incorrect. Tables are a basic unit of storage, composed of rows. Tables do not generate numeric values.

Option 4 is incorrect. A view logically represents subsets of data from one or more tables. Views do not generate numeric values.

Question

Which table names would be acceptable according to the standard rules for naming any Oracle Database 11g object?

Options:

1. 902101_custdata

2. custdata

3. grant

4. this_table_is_very_important_so_please_do_not_delete_it

Answer

The name custdata is acceptable as a table name.

Option 1 is incorrect. Table names and column names must begin with a letter.

Option 2 is correct. The example custdata meets all the requirements for standard naming rules for any Oracle database object.

Option 3 is incorrect. Names must not be an Oracle server-reserved keyword.

Option 4 is incorrect. Table names must be 1–30 characters long.

2. Specifying data types for table columns

In Oracle databases, when you identify a column for a table, you need to provide a data type for the column.

Oracle provides you with these data types:

VARCHAR2(size)

CHAR [(size)]

NUMBER [(p,s)]

DATE

LONG

CLOB

VARCHAR2(size)

The VARCHAR2(size) data type is used for variable-length character data. A maximum size must be specified. The minimum size is 1, and the maximum size is 4,000.

CHAR [(size)]

The CHAR [(size)]data type is used for fixed-length character data of length-size bytes. The default and minimum size is 1, and the maximum size is 2,000.

NUMBER [(p,s)]

The NUMBER [(p,s)] data type is used for numbers having precision p and scale s. Precision is the total number of decimal digits and scale is the number of digits to the right of the decimal point. Precision can range from 1 to 38, and scale can range from –84 to 127.

DATE

The DATE data type is used for date and time values to the nearest second between January 1, 4712 B.C., and December 31, 9999 A.D.

LONG

The LONG data type is used for variable-length character data of up to 2 GB.

CLOB

The CLOB data type is used for character data of up to 4 GB.

Oracle also provides you with these data types:

RAW(size)

LONG RAW

BLOB

BFILE

ROWID

RAW(size)

The RAW(size)data type is used for raw binary data of length size. A maximum size must be specified. The maximum size is 2,000.

LONG RAW

The LONG RAW data type is used for raw binary data of variable length of up to 2 GB.

BLOB

The BLOB data type is used for binary data of up to 4 GB.

BFILE

The BFILE data type is used for binary data stored in an external file of up to 4 GB.

ROWID

The ROWID data type is used for a base-64 number system representing the unique address of a row in its table.

Here are some guidelines for providing a data type for a column:

a LONG column is not copied when a table is created using a subquery

a LONG column cannot be included in a GROUP BY or an ORDER BY clause

only one LONG column can be used per table

no constraints can be defined on a LONG column

you can use a CLOB column rather than a LONG column

Oracle provides you with these datetime data types:

TIMESTAMP

INTERVAL YEAR TO MONTH

INTERVAL DAY TO SECOND

TIMESTAMP

The TIMESTAMP data type enables storage of time as a date with fractional seconds. It stores the year, month, day, hour, minute, and second value of the DATE data type as well as the fractional seconds value. There are several variations of this data type, such as WITH TIMEZONE and WITH LOCALTIMEZONE.

INTERVAL YEAR TO MONTH

The INTERVAL YEAR TO MONTH data type enables storage of time as an interval of years and months. It is used to represent the difference between two datetime values in which the only significant portions are the year and month.

INTERVAL DAY TO SECOND

The INTERVAL DAY TO SECOND data type enables storage of time as an interval of days, hours, minutes, and seconds. It is used to represent the precise difference between two datetime values.

Question

Which data type allows you to enter variable-length character data, which must be defined with a size?

Options:

1. BLOB

2. CHAR

3. DATE

4. VARCHAR2

Answer

The VARCHAR2 data type allows you to enter variable-length character data, which must be defined with a size.

Option 1 is incorrect. The BLOB data type is a binary large object, not a variable-length character data type.

Option 2 is incorrect. The CHAR data type is a fixed-length character data type that must be defined with a size. It is not a variable-length character type.

Option 3 is incorrect. The DATE data type stores date and time values to the nearest second between January 1, 4712 B.C., and December 31, 9999 A. D.

Option 4 is correct. The VARCHAR2 data type allows for variable-length character data. A maximum size must be specified. The minimum size is 1 and the maximum size is 4,000.

Question

Which data type makes use of precision and scale?

Options:

1. DATE

2. LONG

3. NUMBER

4. VARCHAR2

Answer

The NUMBER data type makes use of precision and scale.

Option 1 is incorrect. The DATE data type allows for date and time values to the nearest second between January 1, 4712 B.C., and December 31, 9999 A.D. You would not specify precision and scale with this data type.

Option 2 is incorrect. The LONG data type allows for variable-length character data. You would not specify precision and scale with this data type.

Option 3 is correct. The NUMBER data type allows for a number having precision, p, and scale, s. Precision is the total number of decimal digits and scale is the number of digits to the right of the decimal point.

Option 4 is incorrect. The VARCHAR2 data type allows for variable-length character data. You would not specify precision and scale with this data type.

Summary

Some of the more popular database objects in an Oracle database are tables, views, sequences, indexes, and synonyms. You name database tables and columns according to the standard rules for naming Oracle database objects. You may use quoted identifiers to represent the name of an object. You use the CREATE TABLE statement to create a table and include constraints. You can also create a table based on another table by using a subquery. When you define a table, you can specify that a column should be given a default value by using the DEFAULT option.

When you identify a column for a table, you need to provide a data type for the column. There are several data types available – VARCHAR2(size), CHAR [(size)], NUMBER [(p,s)], DATE, LONG, CLOB, RAW(size), LONG RAW, BLOB, BFILE, and ROWID. The datetime data types are TIMESTAMP, INTERVAL YEAR TO MONTH, and INTERVAL DAY TO SECOND.

Table of Contents

| Top of page |

| Learning objective |

| 1. Creating, defining, and naming a table |

| 2. Specifying data types for table columns |

| Summary |

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Constraints

Learning objective

After completing this topic, you should be able to recognize the steps for using constraints to prevent invalid data entry into tables.

1. Overview of constraints

The Oracle server uses constraints to prevent invalid data entry into tables. You can use constraints to

enforce rules on the table data whenever a row is inserted, updated, or deleted from that table; the constraint must be satisfied for the operation to succeed

prevent deletion of a table if there are dependencies from other tables

provide rules for Oracle tools, such as Oracle Developer

The various data integrity constraints include

NOT NULL

UNIQUE

PRIMARY KEY

FOREIGN KEY

CHECK

NOT NULL

The NOT NULL constraint specifies that the column cannot contain a null value.

UNIQUE

The UNIQUE constraint specifies a column or combination of columns whose values must be unique for all rows in the table.

PRIMARY KEY

The PRIMARY KEY constraint uniquely identifies each row of the table.

FOREIGN KEY

The FOREIGN KEY constraint establishes and enforces a referential integrity between the column and a column of the referenced table such that values in one table match values in another table.

CHECK

The CHECK constraint specifies a condition that must be true.

All constraints are stored in the data dictionary and are easy to refer if you give them a meaningful name. The constraint names must follow the standard object-naming rules, except that the name cannot be the same as another object owned by the same user. If you do not name your constraint, the Oracle server generates a name with the format SYS_Cn, where n is an integer so that the constraint name is unique.

Constraints can be defined at the time of table creation or after the creation of the table. You can define a constraint at the column or table level. Functionally, a table-level constraint is the same as a column-level constraint.

Here is the syntax to define constraints when creating a table.

CREATE TABLE [schema.]table(column datatype [DEFAULT expr][column_constraint],...[table_constraint][,...]);

In this syntax

schema is the same as the owner's name

table is the name of the table

DEFAULT expr specifies a default value to be used if a value is omitted in the INSERT statement

column is the name of the column

datatype is the column's data type and length

column_constraint is an integrity constraint as part of the column definition

table_constraint is an integrity constraint as part of the table definition

CREATE TABLE [schema.]table(column datatype [DEFAULT expr][column_constraint],...[table_constraint][,...]);

You can create constraints either at the column-level or table-level:

column-level

table-level

column-level

You include column-level constraints when the column is defined. The NOT NULL constraints must be defined at the column-level.

Syntaxcolumn [CONSTRAINT constraint_name] constraint_type,

table-level

Table-level constraints are defined at the end of the table definition and must refer to the column or columns on which the constraint pertains in a set of parentheses. Constraints that apply to more than one column must be defined at the table-level.

Syntaxcolumn,...[CONSTRAINT constraint_name] constraint_type(column, ...),"

Functionally, a table-level constraint is the same as a column-level constraint. It is mainly the syntax that differentiates the two.

Constraints are usually created at the same time as the table. You can add constraints to a table after its creation and contraints can also be temporarily disabled. Consider this sample code of a column-level constraint. This code creates a primary key constraint on the EMPLOYEE_ID column of the EMPLOYEES table.

CREATE TABLE employees(employee_id NUMBER(6)CONSTRAINT emp_emp_id_pk PRIMARY KEY,first_name VARCHAR2(20),...);

Here is a sample code that uses the table-level syntax to define the constraint.

CREATE TABLE employees(employee_id NUMBER(6),first_name VARCHAR2(20),...job_id VARCHAR2(10) NOT NULL,CONSTRAINT emp_emp_id_pkPRIMARY KEY (EMPLOYEE_ID));

Question

What constraint type is used to establish and enforce referential integrity?

Options:

1. CHECK

2. FOREIGN KEY

3. PRIMARY KEY

4. UNIQUE

Answer

A FOREIGN KEY constraint is used to establish and enforce referential integrity.

Option 1 is incorrect. A CHECK constraint is not used to establish and enforce referential integrity. It specifies a condition that must be true.

Option 2 is correct. A FOREIGN KEY constraint is used to establish and enforce referential integrity between the column and a column of the referenced table such that values in one table match values in another table.

Option 3 is incorrect. A PRIMARY KEY constraint is not used to establish and enforce referential integrity. It is used to uniquely identify each row of the table.

Option 4 is incorrect. A UNIQUE constraint is used to specify a column or combination of columns whose values must be unique for all rows in the table. It is not used to establish and enforce referential integrity.

2. Types of constraints

The various data integrity constraints include

NOT NULL

UNIQUE

PRIMARY KEY

FOREIGN KEY

CHECK

The NOT NULL constraint ensures that the column contains no null values. By default, columns without the NOT NULL constraint can contain null values. In the EMPLOYEES table, the EMPLOYEE_ID column inherits the NOT NULL constraint as it is defined as a primary key. Otherwise, the FIRST_NAME, LAST_NAME, EMAIL, HIRE_DATE, and JOB_ID columns have the NOT NULL constraint enforced on them.

The UNIQUE key integrity constraint ensures that every value in a column or a set of columns should be unique – that is, no two rows of a table can have duplicate values in a specified column or a set of columns. The column or set of columns included in the definition of the UNIQUE key constraint is called the unique key. If the UNIQUE constraint comprises more than one column, that group of columns is called a composite unique key.

The UNIQUE constraints enable the input of nulls unless you define NOT NULL constraints for the same columns. In fact, any number of rows can include nulls for columns without the NOT NULL constraints because nulls are not considered equal to anything. A null in a column or in all columns of a composite unique key always satisfies the UNIQUE constraint.

Note

Because of the search mechanism for the UNIQUE constraints on more than one column, you cannot have identical values in the non-null columns of a partially null composite UNIQUE key constraint.

The UNIQUE constraint is defined at the column-level or table-level. You can define the constraint at the table-level when you want to create a composite unique key. A composite key is defined when there is not a single attribute that can uniquely identify a row. In that case, you can have a unique key that is composed of two or more columns, the combined value of which is always unique and can identify rows.

Note

The Oracle server enforces the UNIQUE constraint by implicitly creating a unique index on the unique key column or columns.

Here is sample code to apply the UNIQUE constraint on the EMAIL column of the EMPLOYEES table. The name of the constraint is EMP_EMAIL_UK.

CREATE TABLE employees( employee_id NUMBER(6), last_name VARCHAR2(25) NOT NULL, email VARCHAR2(25), salary NUMBER(8,2), commission_pct NUMBER(2,2), hire_date DATE NOT NULL,... CONSTRAINT emp_email_uk UNIQUE(email));

The PRIMARY KEY constraint creates a primary key for the table. Only one primary key can be created for each table. This constraint enforces the uniqueness of the column or

columns and ensures that no column that is part of the primary key can contain a null value.

Note

Because uniqueness is part of the primary key constraint definition, the Oracle server enforces the uniqueness by implicitly creating a unique index on the primary key column or columns.

The FOREIGN KEY or referential integrity constraint designates a column or a combination of columns as a foreign key and establishes a relationship with a primary key or a unique key in the same table or a different table.

In this example, DEPARTMENT_ID is defined as the foreign key in the EMPLOYEES table (dependent or child table). It references the DEPARTMENT_ID column of the DEPARTMENTS table (the referenced or parent table).

You need to consider these guidelines when using the FOREIGN KEY constraint:

a foreign key value must match an existing value in the parent table or be NULL

foreign keys are based on data values and are purely logical, rather than physical, pointers

The FOREIGN KEY constraints can be defined at the column-level or table-level. A composite foreign key must be created by using the table-level definition. This sample code defines the FOREIGN KEY constraint on the DEPARTMENT_ID column of the EMPLOYEES table, using table-level syntax. The name of the constraint is EMP_DEPT_FK.

CREATE TABLE employees( employee_id NUMBER(6), last_name VARCHAR2(25) NOT NULL, email VARCHAR2(25), salary NUMBER(8,2), commission_pct NUMBER(2,2), hire_date DATE NOT NULL,... department_id NUMBER(4), CONSTRAINT emp_dept_fk FOREIGN KEY (department_id) REFERENCES departments(department_id), CONSTRAINT emp_email_uk UNIQUE(email));

The foreign key can also be defined at the column-level, provided the constraint is based on a single column. The syntax differs in that the keywords FOREIGN KEY do not appear.

This is the sample code for defining foreign key constraints at the column-level.

CREATE TABLE employees(...department_id NUMBER(4) CONSTRAINT emp_deptid_fkREFERENCES departments(department_id),...)

The foreign key is defined in the child table and the table containing the referenced column in the parent table. The foreign key is defined using a combination of these keywords:

FOREIGN KEY

REFERENCES

ON DELETE CASCADE

ON DELETE SET NULL

FOREIGN KEY

The FOREIGN KEY keywords are used to define the column in the child table at the table-constraint level.

REFERENCES

The REFERENCES keyword identifies the table and the column in the parent table.

ON DELETE CASCADE

The ON DELETE CASCADE keywords indicate that when a row in the parent table is deleted, the dependent rows in the child table are also deleted.

ON DELETE SET NULL

The ON DELETE SET NULL keywords indicate that when a row in the parent table is deleted, the foreign key values are set to null.

The default behavior is called the restrict rule, which disallows the update or deletion of referenced data. Without the ON DELETE CASCADE or the ON DELETE SET NULL options, the row in the parent table cannot be deleted if it is referenced in the child table.

The CHECK constraint defines a condition that each row must satisfy. The condition can use the same constructs as the query conditions, with these exceptions:

references to the CURRVAL, NEXTVAL, LEVEL, and ROWNUM pseudocolumns

calls to the SYSDATE, UID, USER, and USERENV functions

queries that refer to other values in other rows

A single column can have multiple CHECK constraints that refer to the column in its definition. There is no limit to the number of the CHECK constraints that you can define on a column. The CHECK constraints can be defined at the column-level or table-level. This is sample code to define the CHECK constraints.

Example 1..., salary NUMBER(2)CONSTRAINT emp_salary_minCHECK (salary > 0),...

Example 2CREATE TABLE employees(...salary NUMBER(8,2) CONSTRAINT emp_salary_minCHECK (salary > 0),...

Here is a sample code that creates the EMPLOYEES table in the human resource (HR) schema.

CREATE TABLE employees( employee_id NUMBER(6)CONSTRAINT emp_employee_id PRIMARY KEY, first_name VARCHAR2(20), last_name VARCHAR2(25)CONSTRAINT emp_last_name_nn NOT NULL, email VARCHAR2(25)CONSTRAINT emp_email_nn NOT NULLCONSTRAINT emp_email_uk UNIQUE, phone_number VARCHAR2(20), hire_date DATECONSTRAINT emp_hire_date_nn NOT NULL, job_id VARCHAR2(10)CONSTRAINT emp_job_nn NOT NULL, salary NUMBER(8,2)CONSTRAINT emp_salary_ck CHECK (salary>0), commission_pct NUMBER(2,2), manager_id NUMBER(6)CONSTRAINT emp_manager_fk REFERENCESemployees (employee_id), department_id NUMBER(4)CONSTRAINT emp_dept_fk REFERENCESdepartments (department_id));

Question

What statement is true about FOREIGN KEY constraints?

Options:

1. Composite foreign keys must be defined at the column level

2. Foreign keys are purely logical rather than physical pointers

3. Foreign keys can be defined at the column or table constraint level in all cases

4. Foreign keys are defined in the parent table

Answer

Foreign keys are purely logical rather than physical pointers to the data.

Option 1 is incorrect. Composite foreign keys must be created using the table-level definition.

Option 2 is correct. Foreign keys are based on data values and are purely logical rather than physical pointers to the data.

Option 3 is incorrect. The foreign key can be defined at the column level provided that the constraint is based on a single column.

Option 4 is incorrect. The foreign key is defined in the child table and the table containing the referenced column is the parent table.

Question

What keywords are used within a FOREIGN KEY constraint definition to delete dependent rows in child tables when the related row in the parent table is deleted?

Options:

1. FOREIGN KEY

2. ON DELETE CASCADE

3. ON DELETE SET NULL

4. REFERENCES

Answer

You can use the ON DELETE CASCADE keywords to delete dependent rows in child tables when the related row in the parent table is deleted.

Option 1 is incorrect. The FOREIGN KEY keywords are used to define the column in the child table at the table-constraint level, not delete dependent rows.

Option 2 is correct. The ON DELETE CASCADE keywords indicates that when a row in the parent table is deleted, the dependent rows in the child table are also deleted.

Option 3 is incorrect. The ON DELETE SET NULL keywords indicate that when a row in the parent table is deleted, the foreign key values are set to null.

Option 4 is incorrect. The REFERENCES keyword identifies the table and the column in the parent table.

Question

How is the UNIQUE constraint enforced?

Options:

1. By creating an index on the unique key column or columns

2. By matching an existing value in the parent table

3. By not allowing nulls within the unique key column or columns

4. By placing a primary constraint on the unique key column or columns

Answer

The UNIQUE constraint is enforced by creating an index on the unique key column or columns.

Option 1 is correct. The Oracle server enforces the UNIQUE constraint by implicitly creating a unique index on the unique key column or columns.

Option 2 is incorrect. A UNIQUE key integrity constraint requires that every value in a column or a set of columns be unique so that no two rows of a table can have duplicate values in a specified column or a set of columns. It is not enforced by matching an existing value in a parent table.

Option 3 is incorrect. The UNIQUE constraints enable the input of nulls unless you also define NOT NULL constraints for the same column.

Option 4 is incorrect. The UNIQUE constraint is not enforced by placing a PRIMARY KEY constraint on the unique key column or columns. It is the PRIMARY KEY constraint that is enforced via an implicit creation of a unique index on the primary key column or columns by the Oracle server.

3. Constraints violation

When you have constraints in place on columns, an error is returned if you try to violate the constraint rule. For example, if you try to update a record with a value that is tied to an integrity constraint, an error is returned.

In this sample code, you will receive the "parent key not found" violation ORA-02291 because department 55 does not exist in the parent table, DEPARTMENTS.

UPDATE employeesSET department_id = 55WHERE department_id = 110;

If you attempt to delete a record with a value that is tied to an integrity constraint, an error is returned. You cannot delete a row that contains a primary key that is used as a foreign key in another table.

This sample code tries to delete department 60 from the DEPARTMENTS table, but it results in an error because that department number is used as a foreign key in the EMPLOYEES table. If the parent record that you attempt to delete has child records, then you receive the "child record found" violation ORA-02292.

DELETE FROM departmentsWHERE department_id = 60;

Here is a sample code that works because there are no employees in department 70.

DELETE FROM departmentsWHERE department_id = 70;

result:1 row deleted

Summary

You can define constraints on a table to prevent invalid data entry. By defining constraints you can enforce rules on the table data whenever a row is inserted, updated, or deleted from that table and prevent deletion of a table if there are dependencies from other tables.

You can define NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, and CHECK constraints on a table. You can define constraints at column-level and table-level at the time of table creation or after the table creation. The column-level constraints are included when the column is defined. The table-level constraints are defined at the end of the table definition and must refer to the column or columns on which the constraint pertains in a set of parentheses.

If you try to violate the constraint rule, you will receive an error.

Table of Contents

| Top of page |


| 1. Overview of constraints |

| 2. Types of constraints |

| 3. Constraints violation |

| Summary |



| Print | Contents |

Using the ALTER TABLE and DROP TABLE Statements

Abstract

This article explains the steps for altering or dropping a table, using the ALTER TABLE statement and the DROP TABLE statement.

ALTER TABLE Statement

After you create a table, you may need to change the table structure for any of these reasons:

you omitted a column

your column definition or its name needs to be changed

you need to remove columns

you want to put the table into the read-only mode

You can do this by using the ALTER TABLE statement.

With Oracle Database 11g, you can specify READ ONLY to place a table in the read-only mode. When the table is in the read-only mode, you cannot issue any Dynamic Manipulation Language (DML) statements that affect the table or any SELECT ... FOR UPDATE statements. You can issue Data Definition Language (DDL) statements as long as they do not modify any data in the table. Operations on indexes associated with the table are allowed when the table is in the read-only mode.

You can specify READ/WRITE to return a read-only table to the read/write mode. Here is some sample code to define the table in read-only mode and then back to read/write mode.

ALTER TABLE employees READ ONLY;

-- perform table maintenance and then

-- return table back to read/write mode

ALTER TABLE employees READ WRITE;

Note : You can drop a table that is in the read-only mode. The DROP command is executed only in the data dictionary, so access to the table contents is not required. The space used by the table will not be reclaimed until the tablespace is made read/write again, and then the required changes can be made to the block segment headers, and so on.

DROP TABLE Statement

The DROP TABLE statement moves a table to the recycle bin or removes the table and all its data from the database entirely. Unless you specify the PURGE clause, the DROP TABLE statement does not result in space being released back to the tablespace for use by other objects, and the space continues to count towards the user's space quota. Dropping a table invalidates the dependent objects and removes object privileges on the table.

When you drop a table, the database loses all the data in the table and all the indexes associated with it. In this example, the dept80 table is moved to the recycle bin.

DROP TABLE dept80;

This is the syntax of the DROP TABLE statement.

DROP TABLE table [PURGE]

In this syntax, table is the name of the table.

You need to consider these guidelines when deleting a table:

all the data is deleted from the table

any views and synonyms remain, but are invalid

any pending transactions are committed

only the creator of the table or a user with the DROP ANY TABLE privilege can remove a table

Note : Use the FLASHBACK TABLE statement to restore a dropped table from the recycle bin.

Summary

After you create a table, you can change the table structure by using the ALTER TABLE statement. You can move a table to the recycle bin using the DROP TABLE statement or remove the table and all its data from the database entirely by using the DROP TABLE [PURGE] statement.

Table of Contents

| Top of page |

| Abstract |

| ALTER TABLE Statement |

| DROP TABLE Statement |

| Summary |




Views

Learning objective

After completing this topic, you should be able to identify the steps for creating and manipulating views to view or hide a table's data.

1. Creating views

In addition to tables, there are several other objects in a database. These include views, sequences, indexes, and synonyms.

Each database object performs a specific function.

Table

View

Sequence

Index

Synonym

Table

A table is a basic unit of storage in a database and is composed of rows.

View

A view logically represents subsets of data from one or more tables. With views, you can present and hide data from the tables.

Sequence

A sequence generates numeric values. Many applications require the use of unique numbers as primary key values. You can either build code into the application to handle this requirement or use a sequence to generate unique numbers.

Index

If you want to improve the performance of data retrieval queries, you should consider creating an index. You can also use indexes to enforce uniqueness on a column or a collection of columns.

Synonym

You can provide alternative names for objects by using synonyms.

You can present logical subsets or combinations of data by creating views of tables. A view is a logical table based on a table or another view. A view contains no data of its own, but is like a window through which data from tables can be viewed or changed. The tables on which a view is based are called base tables. The view is stored as a SELECT statement in the data dictionary.

The various advantages of views include:

restricting data access

making complex queries easy

providing data independence

presenting different views of the same data

restricting data access

Views restrict access to the data because they display selected columns from the table.

making complex queries easy

Views can be used to make simple queries to retrieve the results of complicated queries. For example, views can be used to query information from multiple tables without the user knowing how to write a join statement.

providing data independence

Views provide data independence for ad hoc users and application programs. One view can be used to retrieve data from several tables.

presenting different views of the same data

Views provide groups of users access to data according to their particular criteria and therefore, present different views of the same data.

There are two classifications for views and the basic difference is related to the DML (INSERT, UPDATE, and DELETE) operations:

simple

complex

simple

A simple view:

derives data from only one table

contains no functions or groups of data

can perform DML operations through the view

complex

A complex view:

derives data from many tables

contains functions or groups of data

does not always allow DML operations through the view

You can create a view by embedding a subquery in the CREATE VIEW statement. This is the syntax for this statement.

CREATE [OR REPLACE] [FORCE|NOFORCE] VIEW view [(alias[, alias]...)]AS subquery[WITH CHECK OPTION [CONSTRAINT constraint]][WITH READ ONLY [CONSTRAINT constraint]];

Note

In SQL Developer, click the Run Script icon or press the F5 key to run the data definition language (DDL) statements. The feedback messages are displayed on the Script Output tabbed page.

In the syntax for the CREATE VIEW statement, various keywords and parameters are specified:

OR REPLACE

FORCE

NOFORCE

view

alias

subquery

WITH CHECK OPTION

constraint

WITH READ ONLY

OR REPLACE

The OR REPLACE keywords recreate the view if it already exists.

FORCE

The FORCE keyword creates the view regardless of whether or not the base tables exist.

NOFORCE

The NOFORCE keyword creates the view only if the base tables exist. This is the default option.

view

The view parameter specifies the name of the view.

alias

The alias parameter specifies names for the expressions selected by the view's query. The number of aliases must match the number of expressions selected by the view.

subquery

The subquery parameter specifies a complete SELECT statement. You can use aliases for the columns in the SELECT list.

WITH CHECK OPTION

The WITH CHECK OPTION keywords specify that only those rows that are accessible to the view can be inserted or updated.

constraint

The constraint parameter specifies the name assigned to the CHECK OPTION constraint.

WITH READ ONLY

The WITH READ ONLY keywords ensure that no DML operations can be performed on this view.

Here is a sample code that creates a view that contains the employee number, last name, and salary for each employee in department 80.

CREATE VIEW empvu80AS SELECT employee_id, last_name, salaryFROM employeesWHERE department_id = 80;

You can display the structure of the view using the DESCRIBE command.

DESCRIBE empvu80

These are the guidelines for using views:

the subquery that defines a view can contain complex SELECT syntax, including joins, groups, and subqueries

if you do not specify a constraint name for the view created with the WITH CHECK OPTION, the system assigns a default name in the SYS_Cn format

you can use the OR REPLACE option to change the definition of the view without dropping and recreating it, or re-granting the object privileges previously granted on it

You can control the column names by including column aliases in the subquery.

This sample code creates a view containing the employee number (EMPLOYEE_ID) with the alias ID_NUMBER, name (LAST_NAME) with the alias NAME, and annual salary (SALARY) with the alias ANN_SALARY for every employee in department 50.

CREATE VIEW salvu50AS SELECT employee_id ID_NUMBER, last_name NAME,salary*12 ANN_SALARYFROM employeesWHERE department_id = 50;

Alternatively, you can use an alias after the CREATE statement and before the SELECT subquery. The number of aliases listed must match the number of expressions selected in the subquery.

CREATE OR REPLACE VIEW salvu50 (ID_NUMBER, NAME, ANN_SALARY)AS SELECT employee_id, last_name, salary*12FROM employeesWHERE department_id = 50;

Question

Which Oracle Database 11g object is a logical table based on a table or view?

Options:

1. An index

2. A sequence

3. A synonym

4. A view

Answer

A view is a logical table based on a table or view.

Option 1 is incorrect. An index improves the performance of data retrieval queries. Indexes are not logical tables based on a table or view.

Option 2 is incorrect. Sequences are used to generate numeric values. Sequences are not logical tables based on a table or view.

Option 3 is incorrect. Synonyms are used to give alternative names to objects. Synonyms are not logical tables based on a table or view.

Option 4 is correct. A view logically represents subsets of data from one or more tables or views.

Question

You need to create a view containing the employee number (EMPLOYEE_ID) with the alias ID_NUMBER, name (LAST_NAME) with the alias NAME, and the annual salary (SALARY) with the alias ANN_SALARY for every employee in department 50.

Which queries would successfully create this view?

Options:

1. CREATE VIEW salvu50 AS SELECT employee_id ID_NUMBER, last_name NAME, salary*12 ANN_SALARY FROM employees WHERE department_id = 50;

2. CREATE VIEW salvu50 (employee_id, last_name, salary*12) AS SELECT ID_NUMBER, NAME, ANN_SALARY FROM employees WHERE department_id = 50;

3. CREATE VIEW salvu50 (ID_NUMBER, FIRST_NAME, LAST_NAME, ANN_SALARY) AS SELECT employee_id, name, salary*12 FROM employees WHERE department_id = 50;

4. CREATE VIEW salvu50 (ID_NUMBER, NAME, ANN_SALARY) AS SELECT employee_id, last_name, salary*12 FROM employees WHERE department_id = 50;

Answer

These are the queries that can create a view with the required columns.

Option 1 is correct. You can control the column names by including column aliases in the subquery.

Option 2 is incorrect. The column names cannot be listed after the CREATE statement and before the SELECT subquery. It is the aliases that can be listed in this section.

Option 3 is incorrect. The number of aliases listed must match the number of expressions selected in the subquery. There are four aliases and only three expressions in the subquery.

Option 4 is correct. You can use an alias after the CREATE statement and before the SELECT subquery.

2. Modifying and removing views

You can retrieve data from a view as you would from any table. You can display either the contents of the entire view or just specific rows and columns.

SELECT *FROM salvu50;

With the OR REPLACE option, a view can be created even if one exists with this name already, thus replacing the old version of the view for its owner. This means that the view can be altered without dropping, recreating, and re-granting object privileges.

CREATE OR REPLACE VIEW empvu80(id_number, name, sal, department_id)AS SELECT employee_id, first_name || ' '|| last_name, salary, department_idFROM employeesWHERE department_id = 80;

Note

When assigning column aliases in the CREATE OR REPLACE VIEW clause, remember that the aliases are listed in the same order as the columns in the subquery.

Here is a sample code that creates a complex view of department names, minimum salaries, maximum salaries, and the average salaries by department. Note that alternative names have been specified for the view. This is a requirement if any column of the view is derived from a function or an expression.

CREATE OR REPLACE VIEW dept_sum_vu(name, minsal, maxsal, avgsal)AS SELECT d.department_name, MIN(e.salary),MAX(e.salary),AVG(e.salary)FROM employees e JOIN departments dON (e.department_id = d.department_id)GROUP BY d.department_name;

You can view the structure of the view by using the DESCRIBE command and display the contents of the view by issuing a SELECT statement.

SELECT *FROM dept_sum_vu;

You can perform DML operations on data through a view if those operations follow certain rules. You can remove a row from a view unless it contains any of these:

group functions

a GROUP BY clause

the DISTINCT keyword

the pseudocolumn ROWNUM keyword

You can also modify data through a view unless it contains columns defined by expressions, such as SALARY * 12.

You can add data through a view unless it contains any of these items: group functions, a GROUP BY clause, the DISTINCT keyword, the pseudocolumn ROWNUM keyword, or columns defined by expressions.

However, you cannot add data to a view if the view contains NOT NULL columns without default values in the base table. All the required values must be present in the view. Remember that you are adding values directly to the underlying table through the view.

It is possible to perform referential integrity checks through views. You can also enforce constraints at the database level. The view can be used to protect data integrity, but the use is very limited.

The WITH CHECK OPTION clause specifies that INSERTs and UPDATEs performed through the view cannot create rows that the view cannot select. Therefore, it enables integrity constraints and data validation checks to be enforced on data being inserted or updated.

CREATE OR REPLACE VIEW empvu20AS SELECT *FROM employeesWHERE department_id = 20WITH CHECK OPTION CONSTRAINT empvu20_ck;

If there is an attempt to perform DML operations on rows that the view has not selected, an error is displayed, along with the constraint name if that has been specified.

In the sample code, no rows are updated because if the department number were to change to 10, the view would no longer be able to see that employee. Therefore, with the WITH CHECK OPTION clause, the view can see only the employees in department 20 and does not allow the department number for those employees to be changed through the view.

UPDATE empvu20SET department_id = 10WHERE employee_id = 201;

You can ensure that no DML operations occur on your view by creating it with the WITH READ ONLY option.

This sample code modifies the EMPVU10 view to prevent any DML operations on the view.

CREATE OR REPLACE VIEW empvu10(employee_number, employee_name, job_title)AS SELECT employee_id, last_name, job_idFROM employeesWHERE department_id = 10WITH READ ONLY;

Any attempt to remove a row from a view with a read-only constraint results in an error. Similarly, any attempt to insert a row or modify a row using the view with a read-only constraint results in the same error.

DELETE FROM empvu10WHERE employee_number = 200;

You use the DROP VIEW statement to remove a view. The statement removes the view definition from the database. However, dropping views has no effect on the tables on which the view was based.

On the other hand, views or other applications based on the deleted views become invalid. Only the creator or a user with the DROP ANY VIEW privilege can remove a view. This is the syntax for the DROP VIEW statement, in which view is the name of the view.

DROP VIEW view;

This sample code removes the view definition from the database.

DROP VIEW empvu80;

Question

Which clause in a CREATE VIEW statement prevents INSERT, UPDATE, and DELETE statements from being performed on the view?

Options:

1. FORCE

2. OR REPLACE

3. WITH CHECK OPTION

4. WITH READ ONLY

Answer

The WITH READ ONLY clause in a CREATE VIEW statement prevents INSERT, UPDATE, and DELETE statements from being performed on the view.

Option 1 is incorrect. The FORCE clause is used to create the view regardless of whether or not the base tables exist.

Option 2 is incorrect. The OR REPLACE clause is used to recreate the view if it already exists.

Option 3 is incorrect. The WITH CHECK OPTION clause specifies that only those rows that are accessible to the view can be inserted or updated.

Option 4 is correct. The WITH READ ONLY clause when used within the CREATE VIEW statement ensures that no DML operation can be performed on the view.

Summary

Various database objects include views, sequences, indexes, and synonyms. A view is a logical table based on a table or another view. It enables you to restrict data access, make complex queries easy, provide data independence, and present different views of the same data. You can create a view by embedding a subquery in the CREATE VIEW statement and display the structure of the view by using the DESCRIBE command. You can control the column names by including column aliases in the subquery.

You can retrieve data from a view as you would from any table and display either the contents of the entire view or just specific rows and columns. You can also perform referential integrity checks and enforce constraints at the database level. You use the DROP VIEW statement to remove a view.

Table of Contents

| Top of page |


| 1. Creating views |

| 2. Modifying and removing views |

| Summary |




Sequences, Indexes, and Synonyms

Learning objective

After completing this topic, you should be able to recognize the steps for using sequences to create integers, using indexes to improve the performance of queries, and using synonyms to give a table an alternative name.

1. Understanding sequences

A sequence is a user-created database object that generates integer values. You can create sequences and then use them to generate numbers. A sequence can be shared by multiple users to generate integers.

You can also define a sequence to generate unique values or to recycle and use the same numbers again.

A typical use for sequences is to create a primary key value, which must be unique for each row. A sequence is generated and incremented or decremented by an internal Oracle routine. This can be a time-saving object because it can reduce the amount of application code needed to write a sequence generating routine.

Sequence numbers are stored and generated independent of tables. Therefore, the same sequence can be used for multiple tables.

You can automatically generate sequential numbers using the CREATE SEQUENCE statement.

CREATE SEQUENCE sequence[INCREMENT BY n][START WITH n][{MAXVALUE n | NONMAXVALUE}][{MINVALUE n | NONMINVALUE}] [{CYCLE | NOCYCLE}][{CACHE n | NOCACHE}];

The various components of the syntax for the CREATE SEQUENCE statement are

sequence

INCREMENT BY n

START WITH n

MAXVALUE n

NONMAXVALUE

MINVALUE n

NONMINVALUE

CYCLE | NOCYCLE

CACHE n | NOCACHE

sequence

The component sequence is the name of the sequence generator.

INCREMENT BY n

INCREMENT BY n specifies the interval between sequence numbers, where n is an integer. If this clause is omitted, the sequence increments by 1.

START WITH n

START WITH n specifies the first sequence number to be generated. If this clause is omitted, the sequence starts with 1.

MAXVALUE n

MAXVALUE n specifies the maximum value the sequence can generate.

NONMAXVALUE

NONMAXVALUE specifies a maximum value of 10^27 for an ascending sequence and –1 for a descending sequence. This is the default option.

MINVALUE n

MINVALUE n specifies the minimum sequence value.

NONMINVALUE

NONMINVALUE specifies a minimum value of 1 for an ascending sequence and –(10^26) for a descending sequence. This is the default option.

CYCLE | NOCYCLE

CYCLE | NOCYCLE specifies whether the sequence continues to generate values after reaching its maximum or minimum value. NOCYCLE is the default option.

CACHE n | NOCACHE

CACHE n | NOCACHE specifies how many values the Oracle server preallocates and keeps in memory. By default, the Oracle server caches 20 values.

This sample code creates a sequence named DEPT_DEPTID_SEQ to be used for the DEPARTMENT_ID column of the DEPARTMENTS table. The sequence starts at 120, does not allow caching, and does not cycle.

CREATE SEQUENCE dept_deptid_seq INCREMENT BY 10

START WITH 120 MAXVALUE 9999 NOCACHE NOCYCLE;

CREATE SEQUENCE succeeded

You should not use the CYCLE option if the sequence is used to generate primary key values, unless you have a reliable mechanism that purges old rows faster than the sequence cycles.

Note

The sequence is not tied to a table. Generally, you should name the sequence after its intended use. However, the sequence can be used anywhere regardless of its name.

After you create your sequence, it generates sequential numbers for use in your tables. You reference the sequence values using these pseudocolumns:

NEXTVAL

CURRVAL

NEXTVAL

The NEXTVAL pseudocolumn is used to extract successive sequence numbers from a specified sequence. You must qualify NEXTVAL with the sequence name. When you reference sequence.NEXTVAL, a new sequence number is generated and the current sequence number is placed in CURRVAL.

CURRVAL

The CURRVAL pseudocolumn is used to refer to a sequence number that the current user has just generated. However, NEXTVAL must be used to generate a sequence number in the current user's session before CURRVAL can be referenced. You must qualify CURRVAL with the sequence name. When you reference sequence.CURRVAL, the last value returned to that user's process is displayed.

You can use NEXTVAL and CURRVAL in these contexts:

the SELECT list of a SELECT statement that is not part of a subquery

the SELECT list of a subquery in an INSERT statement

the VALUES clause of an INSERT statement

the SET clause of an UPDATE statement

You cannot use NEXTVAL and CURRVAL in these contexts:

the SELECT list of a view

a SELECT statement with the DISTINCT keyword

a SELECT statement with GROUP BY, HAVING, or ORDER BY clauses

a subquery in a SELECT, DELETE, or UPDATE statement

the DEFAULT expression in a CREATE TABLE or ALTER TABLE statement

This sample code inserts a new department in the DEPARTMENTS table. It uses the DEPT_DEPTID_SEQ sequence to generate a new department number.

INSERT INTO departments(department_id, department_name, location_id)VALUES (dept_deptid_seq.NEXTVAL, 'Support', 2500);

You can view the current value of the sequence using the sequence_name.CURRVAL.

SELECT dept_deptid_seq.CURRVALFROM dual;

Say you now want to hire employees to staff the new department.

This is the INSERT statement to be executed for all new employees. This sample code assumes that a sequence called EMPLOYEE_SEQ has already been created to generate new employee numbers.

INSERT INTO employees (employee_id, department_id, ...)VALUES (employees_seq.NEXTVAL, dept_deptid_seq .CURRVAL, ...);

You can cache sequences in memory to provide faster access to sequence values. The cache is populated the first time you refer to the sequence.

Each request for the next sequence value is retrieved from the cached sequence. After the last sequence value is used, the next request for the sequence pulls another cache of sequences into memory.

Although sequence generators issue sequential numbers without gaps, these might be the instances for the occurance of a gap:

roll back of a statement

system crash

same sequence being used for multiple tables

roll back of a statement

If you roll back a statement containing a sequence, the number is lost, and a gap occurs in the generation of sequential numbers. This is due to sequence generators issuing sequential numbers independent of a commit or rollback.

system crash

If the sequence caches values in memory those values are lost if the system crashes and a gap occurs in the sequence.

same sequence being used for multiple tables

Because sequences are not tied directly to tables, the same sequence can be used for multiple tables. However, if you do so, each table can contain gaps in the sequential numbers.

If you reach the MAXVALUE limit for your sequence, no additional values from the sequence are allocated and you will receive an error indicating that the sequence exceeds the MAXVALUE.

To continue to use the sequence, you can modify it by using the ALTER SEQUENCE statement. In this syntax, sequence is the name of the sequence generator.

ALTER SEQUENCE sequence[INCREMENT BY n][{MAXVALUE n | NOMAXVALUE}][{MINVALUE n | NOMINVALUE}][{CYCLE | NOCYCLE}][{CACHE n | NOCACHE}];

You must be the owner of the sequence and have the ALTER privilege for the sequence to modify it or the DROP ANY SEQUENCE privilege to remove it.

Also, only future sequence numbers are affected by the ALTER SEQUENCE statement.

The START WITH option cannot be changed using ALTER SEQUENCE. The sequence must be dropped and recreated to restart the sequence at a different number.

To modify a sequence, some validation is also performed. For example, a new MAXVALUE that is less than the current sequence number cannot be imposed.

ALTER SEQUENCE dept_deptid_seq INCREMENT BY 20 MAXVALUE 90 NOCACHE NOCYCLE;

To remove a sequence you can use a DROP SEQUENCE statement.

DROP SEQUENCE dept_deptid_seq;

Question

Which statement is true for sequences?

Options:

1. All sequence numbers are affected by ALTER SEQUENCE statements

2. Modified sequences will allow a new MAXVALUE less than the current sequence number

3. Only the owner of a sequence can modify it

4. The START WITH option cannot be changed using ALTER SEQUENCE

Answer

In sequences, the START WITH option cannot be changed using ALTER SEQUENCE.

Option 1 is incorrect. Only future sequence numbers are affected by the ALTER SEQUENCE statement.

Option 2 is incorrect. Some validation is performed when modifying sequences. A new MAXVALUE less than the current sequence number cannot be imposed.

Option 3 is incorrect. Anyone with the ALTER privilege for a sequence can modify it, as can the owner.

Option 4 is correct. The START WITH option cannot be changed using ALTER SEQUENCE. The sequence must be dropped and recreated to restart the sequence at a different number.

Question

Which statements are true for a sequence created with this code?

CREATE SEQUENCE dept_deptid_seq MAXVALUE 9999 NOCYCLE;

Options:

1. The Oracle server will not cache any values

2. The sequence will increment by 1

3. The sequence will not generate values after reaching 9999

4. The statement will fail because the INCREMENT clause is missing

Answer

For the given code, the sequence will not generate values after reaching 9999 and the sequence will increment by 1.

Option 1 is incorrect. The CACHE clause specifies how many values the Oracle server preallocates and keeps in memory. By default, the Oracle server caches 20 values.

Option 2 is correct. The INCREMENT BY clause specifies the interval between sequence numbers. If omitted, the sequence increments by 1.

Option 3 is correct. The MAXVALUE clause specifies the maximum value the sequence can generate is 9999 in this case.

Option 4 is incorrect. The statement will not fail because of a missing INCREMENT clause. The sequence would use the default value of 1 if the INCREMENT clause is missing.

2. Understanding indexes

Indexes are database objects that you can create to improve the performance of some queries. Indexes can also be created automatically by the server when you create a primary key or a unique constraint.

An Oracle server index is a schema object that can speed up the retrieval of rows by using a pointer. Indexes can be created explicitly or automatically. If you do not have an index on the column, then a full table scan occurs.

An index provides direct and fast access to rows in a table. Its purpose is to reduce the disk I/O by using an indexed path to locate data quickly. An index is used and maintained automatically by the Oracle server. After an index is created, no direct activity is required by the user.

Indexes are logically and physically independent on the table that they index. This means that they can be created or dropped at any time, and have no effect on the base tables or other indexes.

Note

When you drop a table, the corresponding indexes are also dropped.

You can create two types of indexes:

unique index

nonunique index

unique index

The Oracle server automatically creates this index when you define a column in a table to have a PRIMARY KEY or a UNIQUE constraint. The name of the index is the name that is given to the constraint.

You can manually create a unique index, but it is recommended that you create a unique constraint, which implicitly creates a unique index.

nonunique index

This is an index that a user can create. For example, you can create the FOREIGN KEY column index for a join in a query to improve the speed of retrieval.

You create an index on one or more columns by issuing the CREATE INDEX statement.

In this syntax index is the name of the index, table is the name of the table, and column is the name of the column in the table to be indexed.

You specify

UNIQUE to indicate that the value of the column (or columns) upon which the index is based must be unique.

BITMAP to indicate that the index is to be created with a bitmap for each distinct key, rather than indexing each row separately. Bitmap indexes store the row ids associated with a key value as a bitmap.

CREATE [UNIQUE][BITMAP]INDEX indexON table (column[, column]...);

Having more indexes on a table does not produce faster queries. Each data manipulation language (DML) operation that is committed on a table with indexes means that the indexes must be updated. The more indexes that you have associated with a table, the more effort the Oracle server must make to update all the indexes after a DML operation.

You create an index when

a column contains a wide range of values

a column contains a large number of null values

one or more columns are frequently used together in a WHERE clause or a join condition

the table is large and most queries are expected to retrieve less than 2% to 4% of the rows in the table

You do not create an index when:

the columns are not often used as a condition in the query

the table is small or most queries are expected to retrieve more than 2% to 4% of the rows in the table

the table is updated frequently

the indexed columns are referenced as part of an expression

Note

If you want to enforce uniqueness, you should define a unique constraint in the table definition. A unique index is then created automatically.

You cannot modify indexes. To change an index, you must drop it and then re-create it. Remove an index definition from the data dictionary by issuing the DROP INDEX statement.

To drop an index, you must be the owner of the index or have the DROP ANY INDEX privilege. In the syntax, index is the name of the index. For example, to remove the emp_last_name_idx index from the data dictionary you use this code.

DROP INDEX index;

If you drop a table, indexes and constraints are automatically dropped but views and sequences remain.

Question

In which situations would it be advisable to create an index?

Options:

1. When the columns are not often used as a condition in the query

2. When the column contains a wide range of values

3. When the table is large and queries will be very selective

4. When the table is updated frequently

Answer

It is advisable to create an index when the column contains a wide range of values and when the table is large and queries will be very selective.

Option 1 is incorrect. It is advisable to create an index if one or more columns are frequently used together in a WHERE clause or a join operation. If the columns are not often used as a condition in the query, an index should not be created on it.

Option 2 is correct. If the column contains a small number of values it is not a good candidate as an index column. Columns with a wide range of values are good candidates.

Option 3 is correct. Large tables, in which the majority of queries are expected to retrieve less than 2% to 4% of the rows of the table, are good candidates for indexing.

Option 4 is incorrect. Each DML operation that is committed on a table with indexes means the indexes must be updated. So columns that are updated frequently are poor index column candidates.

3. Understanding synonyms

Synonyms are database objects that enable you to call a table by another name. You can create synonyms to give an alternative name to a table.

To refer to a table that is owned by another user, you need to prefix the table name with the name of the user who created it, followed by a period.

Creating a synonym eliminates the need to qualify the object name with the schema and provides you with an alternative name for a table, view, sequence, procedure, or other objects. This method can be especially useful with lengthy object names, such as views.

CREATE [PUBLIC] SYNONYM synonymFOR object;

The various components of the syntax for creating a synomym are

PUBLIC

synonym

object

PUBLIC

PUBLIC creates a synonym that is accessible to all users.

synonym

The synonym component is the name of the synonym to be created.

object

The object component identifies the object for which the synonym is created.

The guidelines for creating synonyms include

an object cannot be contained in a package

a private synonym name must be distinct from all other objects that are owned by the same user

This sample code creates a synonym for the DEPT_SUM_VU view for quicker reference. The database administrator can create a public synonym that is accessible to all users.

CREATE SYNONYM d_sumFOR dept_sum_vu;

This sample code creates a public synonym named DEPT for Alice's DEPARTMENTS table.

CREATE PUBLIC SYNONYM deptFOR alice.departments;

To remove a synonym, you use the DROP SYNONYM statement. Only the database administrator can drop a public synonym.

DROP PUBLIC SYNONYM dept;

Summary

Sequences are user-defined database objects that generate numeric values. You can automatically generate sequential numbers using the CREATE SEQUENCE statement. Sequences can be used to generate primary key values that are unique.

Indexes are database objects used to improve the performance of some queries. An index can also be created automatically by the server when you create a primary key or a unique constraint. It provides direct and fast access to rows in a table and is used to reduce the disk I/O.

Synonyms are database objects that enable you to call a table by another name. You can create synonyms to give an alternative name to database objects.

Table of Contents

| Top of page |


| 1. Understanding sequences |

| 2. Understanding indexes |

| 3. Understanding synonyms |

| Summary |




Creating Tables, Sequences, Synonyms, and Indexes

Learning objective

After completing this topic, you should be able to create a new table and create and use a sequence, index, and synonym.

Exercise overview

In this exercise, you are required to complete and identify DDL statements.

This involves the following tasks:

creating tables

creating views

creating other schema objects such as sequences, synonyms, and indexes

You are a database administrator (DBA) for an Oracle Database 11g database. The HR department wants you to create some new tables in the Oracle database for storing customer information.

Task 1: Creating tables

The HR manager has asked you to create a table that contains information about various departments. The manager has also specified the number and type of columns.

Step 1 of 6

You want to create a new DEPT table with two columns called ID and NAME. There must be a primary key constraint on the ID column. The ID column must accept a maximum of 7 decimal digits. The NAME column must accept entries with a maximum of 25 variable-length characters.

Type the <missing code> to perform this task.

<missing code>(id NUMBER(7)CONSTRAINT department_id_pk PRIMARY KEY, name VARCHAR2(25));

Result

You type this code:CREATE TABLE dept

The PRIMARY KEY constraint is a column or columns to uniquely identify each row in a table. This constraint enforces the uniqueness of the column or columns and ensures that no column that is part of the primary key can contain a null value.

Now you want to populate the DEPT table with information from another table.

Step 2 of 6

You want to populate the DEPT table with data from the DEPARTMENTS table and include only those columns that you need.

Identify the code that you would use to complete this task.

Options:

1. INSERT INTO departmentsSELECT department_id, department_nameFROM dept;

2. INSERT INTO departmentsSELECT department_id, department_name, manager_id, location_idFROM dept;

3. INSERT INTO deptSELECT department_id, department_nameFROM departments;

4. INSERT INTO deptSELECT department_id, department_name, manager_id, location_idFROM departments;

Result

The code that successfully completes this task is

INSERT INTO deptSELECT department_id, department_nameFROM departments;

Option 1 is incorrect. The DEPT table should follow the INSERT INTO statement, and the DEPARTMENTS table should follow the FROM clause.

Option 2 is incorrect. The DEPT table should follow the INSERT INTO statement, and the DEPARTMENTS table should follow the FROM clause. Also, the only

columns that should be included within the SELECT statement are DEPARTMENT_ID and DEPARTMENT_NAME because the DEPT table only includes these two columns.

Option 3 is correct. This statement will populate the DEPT table with data from the two columns DEPARTMENT_ID and DEPARTMENT_NAME found within the DEPARTMENTS table.

Option 4 is incorrect. The only columns that should be included within the SELECT statement are DEPARTMENT_ID and DEPARTMENT_NAME because the DEPT table only includes these two columns.

The HR manager has also asked you to create another table for storing employee information. The manager has specified the number and type of columns to be created in this table.

Step 3 of 6

You need to create a table called EMP with four columns. The ID and the DEPT_ID column should accept a maximum of 7 decimal digits. The LAST_NAME and FIRST_NAME columns must accept entries with a maximum of 25 characters. There is also a FOREIGN KEY constraint on the DEPT_ID column that references the ID column of the DEPT table.

Complete the code to perform this task.

CREATE TABLE empMISSING CODE last_name VARCHAR2(25),first_name VARCHAR2(25),dept_id NUMBER(7)CONSTRAINT emp_dept_id_FK REFERENCES dept (id));

Result

You type this code:(id NUMBER(7),

The FOREIGN KEY establishes and enforces a referential integrity between the column and a column of the referenced table so that the values in one table match the values in another table.

Now you want to change the status of the table to read-only.

Step 4 of 6

You want to change the status of the EMP table to read-only.

Type the code that will alter the status of the EMP table.

Result

You type this code:ALTER TABLE emp READ ONLY;

Because the EMP table is set to READ ONLY, you get the "Update operation not allowed on table" error message when you try to insert a row. Hence, you are not allowed to insert any row into the table because it is assigned a read-only status.

You want to insert some rows in a table. As a result, you now want to change the status of the table to read/write.

Step 5 of 6

You want to revert the status of the EMP table to read/write.

Type the code that enables you to do this.

Result

You type this code:ALTER TABLE emp READ WRITE;

Because the table is assigned a READ WRITE status, you will be allowed to insert a row into the table.

You realize that the table is not required in the database and you want to remove it.

Step 6 of 6

You want to remove the EMP table from the database.

Type the code that enables you to do this.

Result

You type this code:DROP TABLE emp;

You can even drop a table that is in the READ ONLY mode.

Task 2: Creating views

The HR department wants to hide some of the data in a table. It wants you to create a view based on some specific columns from the base table.

Step 1 of 2

The HR department wants to hide some of the data in the EMPLOYEES table. It wants you to create a view called EMPLOYEES_VU based on the employee numbers, employee last names, and department numbers from the EMPLOYEES table. The department also wants the heading for the employee name column to be EMPLOYEE.

Which statement will do this?

Options:

1. CREATE OR REPLACE VIEW employee AS SELECT employee_id, last_name employee, department_idFROM employees;

2. CREATE OR REPLACE VIEW employees_vu AS SELECT employee_id, last_name, department_idFROM employees;

3. CREATE OR REPLACE VIEW employees_vu AS SELECT employee_id, last_name employee, department_id FROM employees;

4. CREATE OR REPLACE VIEW employees_vu AS SELECT employee_id, last_name employee FROM employees;

Result

The statement that performs the required task is

CREATE OR REPLACE VIEW employees_vu AS SELECT employee_id, last_name employee, department_id FROM employees;

Option 1 is incorrect. The view name should be EMPLOYEES_VU, not EMPLOYEE.

Option 2 is incorrect. This statement is missing the alias heading EMPLOYEE for the LAST_NAME column.

Option 3 is correct. This statement will create or replace an existing view called EMPLOYEES_VU that will display data in the EMPLOYEE_ID, LAST_NAME, and DEPARTMENT_ID columns. It will also display the LAST_NAME column as EMPLOYEE.

Option 4 is incorrect. This CREATE VIEW statement is missing the requested DEPARTMENT_ID column in the SELECT statement.

Now you want to create another view that contains some specific columns from a table and label the columns, as specified by the HR department.

Step 2 of 2

You are creating a view named DEPT50 that contains the ids, last names, and department numbers for all employees in department 50. For security purposes, you can not allow an employee to be reassigned to another department. You need to define a CHECK CONSTRAINT called emp_dept_50 to implement this security.

Complete the code to perform this task.

CREATE VIEW dept50 ASSELECT employee_id empno, last_name employee,department_id deptnoFROM employeesWHERE department_id = 50MISSING CODE

Result

You type this code:WITH CHECK OPTION CONSTRAINT emp_dept_50;An error will now occur if someone attempts to reassign an employee to a different department number. The DEPT50 view has been created with the CHECK CONSTRAINT to ensure that the DEPTNO column is protected from being changed.

Task 3: Creating other schema objects

You want to create a sequence for a primary key column of a table.

Step 1 of 3

You need a sequence that can be used with the primary key column of the DEPT table. The sequence should start at 200 and have a maximum value of 1,000.

Your sequence must increment by 10, and you want to name the sequence DEPT_ID_SEQ.

Which statement will do this?

Options:

1. CREATE SEQUENCE dept_id_seqSTART WITH 10INCREMENT BY 200MAXVALUE 1000;

2. CREATE SEQUENCE dept_id_seqSTART WITH 200INCREMENT BY 10MAXVALUE 1000;

3. CREATE SEQUENCE dept_id_seqSTART WITH 200INCREMENT BY 10MINVALUE 1000;

4. CREATE SEQUENCE dept_idSTART WITH 200INCREMENT BY 10MINVALUE 1000;

Result

This statement will perform this task:

CREATE SEQUENCE dept_id_seqSTART WITH 200INCREMENT BY 10MAXVALUE 1000;

Option 1 is incorrect. The sequence should start with 200 and increment by 10, and not start with 10 and increment by 200.

Option 2 is correct. This sequence statement will start at 200, have an increment of 10, and have a maximum value of 1,000.

Option 3 is incorrect. The sequence should have a MAXVALUE of 1,000 and not a MINVALUE of 1,000.

Option 4 is incorrect. The sequence name should be called DEPT_ID_SEQ and not DEPT_ID. The sequence should have a MAXVALUE of 1,000 and not a MINVALUE of 1,000.

The HR department has now asked you to create a nonunique index on the column of a table.

Step 2 of 3

Create a nonunique index on the NAME column in the DEPT table called dept_name_idx.

Type the code that will run this task.

Result

To create a nonunique index on the NAME column in the DEPT table, you type this code:CREATE INDEX dept_name_idx ON dept (name);

You can create an index on one or more columns by issuing the CREATE INDEX statement.

You also want to create a synonym for a table in the database.

Step 3 of 3

You want to create a synonym for the EMPLOYEES table and name it as EMP.

Type the code that will complete this task.

Result

To create a synonym for the EMPLOYEES table, you type this code:CREATE SYNONYM emp FOR EMPLOYEES;

You can create synonyms to give an alternative name to a table by issuing the CREATE SYNONYM statement.

Table of Contents

| Top of page |


| Exercise overview |

| Task 1: Creating tables |

| Task 2: Creating views |

| Task 3: Creating other schema objects |



oracle database 11g using ddl views sequences indexes and synonyms

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