sql ch 4

SQL Basics – Ch 4 – Relational Databases

Prof. Mukesh N. Tekwani

SQL – RELATIONAL DATABASES 1. What is a SQL statement?

An SQL statement requests an specific action from the DBMS. This action may be

creating a new table, inserting a new record, retrieving data, deleting a record, or

modifying a database.

The main structure of an SQL statement is shown below:

Every SQL statement begins with a keyword that describes what the statement does.

CREATE, INSERT, DELETE, and COMMIT are typical verbs. The statement continues

with one or more clauses. A clause may specify the data to be acted upon by the statement

or provide more detail about what the statement is supposed to do. Every clause also

begins with a keyword, such as WHERE, FROM, INTO, and HAVING. Some clauses are

optional; others are required. Many clauses contain table or column names; some may

contain additional keywords, constants, or expressions.

Keywords will always be shown in uppercase e..g., SELECT, FROM, etc

Variable items such as table name, are shown in lowercase letters.

2. What are the different types of SQL statements?

SQL statements can be divided into the following categories:.

(i) Data Manipulation Statements - SELECT, INSERT, DELETE, UPDATE

(ii) Data Definition statements – CREATE TABLE, DROP TABLE, ALTER

TABLE, CREATE VIEW, DROP VIEW, CREATE INDEX, DROP INDEX,

CREATE SCHEMA, DROP SCHEMA, CREATE DOMAIN, ALTER DOMAIN,

DROP DOMAIN

(iii) Access Control – GRANT, REVOKE

SQL Basics– Ch 4 – Relational Databases

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(iv) Transaction Control – COMMIT, ROLLBACK, SET TRANSACTION

(v) Programmatic SQL statements – DECLARE, EXPLAIN, OPEN, CLOSE,

FETCH, PREPARE, EXECUTE, DESCRIBE.

3. What are table names and column names?

(i) The objects in a SQL-based database are identified by assigning them unique names. Names are used in SQL statements to identify the database object on which the statement should act.

(ii) The most fundamental named objects in a relational database are table names (which identify tables), column names (which identify columns), and user names (which identify users of the database).

(iii) The original ANSI/ISO standard specified that SQL names must contain 1 to 18

characters, must begin with a letter, and may not contain any spaces or special punctuation characters. The SQL2 standard increased the maximum to 128 characters.

Table Name: When we specify a table name in a SQL statement, SQL assumes that you are referring to one of your own tables (that is, a table that you created). The table names must be short and descriptive.

Column Name: When we specify a column name in a SQL statement, SQL can normally determine from the context which column we want. However, if the statement involves two columns with the same name from two different tables, we must use a qualified column name to uniquely identify the column we want. A qualified column name specifies the name of the table containing the column and the name of the column, separated by a period (.). For example, the column named SALES in the SALESREPS table has the qualified column name:

SALESREPS.SALES Qualified column names can generally be used in a SQL statement wherever a simple (unqualified) column name can appear.

4 Describe the different data types used in SQL.

The ANSI SQL standard specifies the various data types that can be stored in a SQL-based database and manipulated by the SQL language. These data types are:

(i) Integer - Columns holding this type of data store counts, quantities, ages, and so on. Integer columns are also used to contain I.D. numbers, such as customer, employee, and order numbers.

INT or INTEGER (ii) Decimal numbers. Columns with this data type store numbers that have

fractional parts and must be calculated exactly, such as rates and percentages. They are also used to store money amounts.



(iii) Floating point numbers. Columns with this data type are used to store

scientific numbers that can be calculated approximately, such as weights and distances. Floating point numbers can represent a larger range of values than decimal numbers but can produce round-off errors in computations.

FLOAT (precision) (iv) Fixed-length character strings. Columns holding this type of data store names

of people and companies, addresses, descriptions, and so on.

CHAR(len) or CHARATER (len)

(v) Variable-length character strings. This data type allows a column to store

character strings that vary in length from row to row, up to some maximum length.

VARCHAR (len) or CHARACTER VARYING (len) or CHAR VARYING (len)

(vi) Money amounts. Many SQL products support a MONEY or CURRENCY

type, which is usually stored as a decimal or floating point number.

(vii) Dates and times. SQL can also support date/time values. Various combinations

of dates, times, timestamps, time intervals, and date/time arithmetic are supported.

DATE , TIME (viii) Boolean data. Some SQL products support logical (TRUE or FALSE) values

as an explicit type, and some permit logical operations (comparison, AND/OR, and so on) on the stored data within SQL statements.

(ix) Long text. Many SQL-based databases support columns that store long text

strings (typically up to 32,000 or 65,000 characters, and in some cases even larger). This allows the database to store entire documents, product descriptions, technical papers, resumes, and similar unstructured text data. The DBMS usually restricts the use of these columns in interactive queries and searches.

(x) Unstructured byte streams. Several DBMS products allow unstructured,

variable-length sequences of bytes to be stored and retrieved. Columns containing this data are used to store compressed video images, executable code, and other types of unstructured data.

5 What are constants in SQL? Explain the different types of constants used in SQL.

In some SQL statements a numeric, character, or date data value must be expressed in text


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form. For example, in this INSERT statement, which adds a salesperson to the database:

INSERT INTO SALESREPS (EMPL_NUM, NAME, QUOTA,

HIRE_DATE, SALES)

VALUES (115, 'Dennis Irving', 175000.00, '21-JUN-90',

0.00)

the value for each column in the newly inserted row is specified in the VALUES clause.

Constant data values are also used in expressions, such as in this SELECT statement:

SELECT CITY

FROM OFFICES

WHERE TARGET > (1.1 * SALES) + 10000.00

Numeric Constants:

Integer and decimal constants are written as ordinary decimal numbers in SQL statements,

with an optional leading plus or minus sign.

21 -375 2000.00 +497500.8778

We must not put a comma between the digits of a numeric constant.

Floating point constants (also called approximate numeric literals) are specified using the

E notation. Here are some valid SQL floating point constants:

1.5E3 -3.14159E1 2.5E-7 0.783926E21

String Constants:

These contain character data enclosed within single quotes mark. For example,

'New Delhi.' 'New York' 'Western'

If a single quote is to be included in the constant text, it is written within the constant as

two consecutive single quote characters. Thus this constant value:

'I can''t' becomes the seven-character string "I can't".

Date and Time Constants:

In SQL constant values for dates, times, and time intervals are specified as string

constants. Here are some examples of legal SQL Server date constants:

March 15, 1990 Mar 15 1990 3/15/1990 3-15-90 1990 MAR 15

6 What are symbolic constants in SQL?

SQL language includes special symbolic constants that return data values maintained by

the DBMS itself. For example, the symbolic constant CURRENT_DATE gives the value



of the current date and can be used in queries such as the following, which lists the

salespeople whose hire date is still in the future.

SELECT NAME, HIRE_DATE

FROM SALESREPS

WHERE HIRE_DATE > CURRENT_DATE

A symbolic constant can appear in a SQL statement anywhere that an ordinary constant of

the same data type could appear.

7 How are expressions used in SQL statements?

Expressions are used in the SQL to calculate values that are retrieved from a database and

to calculate values used in searching the database. For example, this query calculates the

sales of each office as a percentage of its target:

SELECT CITY, TARGET, SALES, (SALES/TARGET) * 100

FROM OFFICES

and this query lists the offices whose sales are more than $50,000 over target:

SELECT CITY

FROM OFFICES

HERE SALES > TARGET + 50000.00

The ANSI SQL standard specifies four arithmetic operations that can be used in

expressions: addition , subtraction), multiplication, and division.

8 Explain automatic data type conversion in SQL statements.

The ANSI standard specifies automatic data type conversion from integers to decimal

numbers, and from decimal numbers to floating point numbers. We can thus mix these

data types in a numeric expression.

9 What are built-in functions in SQL?

Most SQL implementations include a number of useful built-in functions. These facilities

often provide data type conversion facilities. For example, built-in MONTH() and YEAR()

functions take a DATE or TIMESTAMP value as their input and return an integer that is

the month or year portion of the value. This query lists the name and month of hire for

each salesperson in the sample database:

SELECT NAME, MONTH(HIRE_DATE)


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FROM SALESREPS

and this one lists all salespeople hired in 1988:

SELECT NAME, MONTH(HIRE_DATE)

FROM SALESREPS

WHERE YEAR(HIRE_DATE) = 1988

Built-in functions also are often used for data reformatting. TO_CHAR() function, for

example, takes a DATE data type and a format specification as its arguments and returns a

string containing a formatted version of the date. In the results produced by this query:

SELECT NAME, TO_CHAR(HIRE_DATE,'DAY MONTH DD, YYYY')

FROM SALESREPS

The hire dates will all have the format "Wednesday June 14, 1989" because of the built-in

function.

10 Explain the term “primary key”.

(i) The rows of a relational table are unordered, and so we cannot select a specific row

by its position in the table. There is no "first row," "last row," or "thirdth row" of a

table.

(ii) In a well-designed relational database every table has some column or combination

of columns whose values uniquely identify each row in the table. This column (or

columns) is called the primary key of the table.

(iii) E.g., in a list of students, we cannot use the lastname field as the primary key since

two or more students may have the same lastname. We may combine the lastname

and firstname and create a primary key. But in practice, "Roll number," such as is

often chosen as primary keys. In a table of orders placed by customers, the

ORDERNO can be used as a primary key.

(iv) The primary key has a different unique value for each row in a table, so no two rows

of a table with a primary key are exact duplicates of one another. A table where

every row is different from all other rows is called a relation in mathematical terms.

The name "relational database" comes from this term, because relations (tables with

distinct rows) form the basis of a relational database.

The following figure shows a table with two columns being used for a primary key.

This key is called a composite key.



11 Explain the term “foreign key”.

(i) A column in one table whose value matches the primary key in some other table is

called a foreign key.

(ii) In the figure shown below, the REP_OFFICE column is a foreign key for the

OFFICES table. Although REP_OFFICE is a column in the SALESREPS table, the

values that this column contains are office numbers. They match values in the

OFFICE column, which is the primary key for the OFFICES table. Together, a

primary key and a foreign key create a parent/child relationship between the tables

that contain them.

(iii) Just as a combination of columns can serve as the primary key of a table, a foreign

key can also be a combination of columns. In fact, the foreign key will always be a

compound (multi-column) key when it references a table with a compound primary

key. The number of columns and the data types of the columns in the foreign key

and the primary key must be identical to one another.

(iv) A table can contain more than one foreign key if it is related to more than one other

table. Figure below shows the three foreign keys in the ORDERS table of the

sample database:


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� The CUST column is a foreign key for the CUSTOMERS table, relating each order

to the customer who placed it.

� The REP column is a foreign key for the SALESREPS table, relating each order to

the salesperson who took it.

� The MFR and PRODUCT columns together are a composite foreign key for the

� PRODUCTS table, relating each order to the product being ordered.

(v) Foreign keys are an important part of the relational model because they create

relationships among tables in the database.

12 What are Codd’s twelve rules for databases?

Rule 1: The information rule. All information in a relational database is represented

explicitly at the logical level and in exactly one way—by values in tables.

Rule 2: Guaranteed access rule. Each and every datum (atomic value) in a relational

database is guaranteed to be logically accessible by using a combination of table name,

primary key value, and column name. This rule stresses the importance of primary key to

locate data. The table name locates the correct table, the column name finds the correct

column, and the primary key value finds the row containing an individual data item of

interest.

Rule 3: Systematic treatment of null values. Null values (distinct from an empty character

string or a string of blank characters and distinct from zero or any other number) are

supported in a fully relational DBMS for representing missing information and

inapplicable information in a systematic way, independent of the data type. Rule 3 requires



support for missing data through NULL values.

Rule 4: Dynamic online catalog based on the relational model. The database description

is represented at the logical level in the same way as ordinary data, so that authorized users

can apply the same relational language to its interrogation as they apply to the regular data.

Rule 5: Comprehensive data sublanguage rule. A relational system may support several

languages. However, there must be at least one language whose statements can be

expressed, by some well-defined syntax. This language should also support the following

activities:

• Data definition

• View definition

• Data manipulation (interactive and by program)

• Integrity constraints

• Authorization

• Transaction boundaries (begin, commit, and rollback)

What this rule means is that the RDBMS should have its own extension of SQL. This

extension should support the above activities

Rule 6: View updating rule. All views that are theoretically updateable are also updateable

by the system.

Rule 7: High-level insert, update, and delete. The capability of handling a base relation or

a derived relation as a single operand applies not only to the retrieval of data but also to the

insertion, update, and deletion of data. The RDBMS should not only support retrieval of

data as relational sets, but should also support insertion, deletion, and updation of data as a

relational set. If the database uses a single record at a time and procedural techniques to

manipulate the data, then it cannot be called a relational database.

Rule 8: Physical data independence. Application programs are not disturbed if any

changes are made in either storage representations or access methods.

Rule 9: Logical data independence. Application programs and terminal activities remain

logically unimpaired when information preserving changes of any kind that theoretically

permit unimpairment are made to the base tables. Thus, this rule allows dynamic changes

in the logical database design by splitting or joining base tables, provided it does not lead

to loss of information.

Rule 10: Integrity independence. Integrity constraints specific to a particular relational

database must be definable in the relational data sublanguage and storable in the catalog,


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not in the application programs. Every database must support the following rules:

� Entity integrity: No component of a primary key can have a NULL value.

� Referential Integrity: For every unique ‘non-null’ foreign key value, there should

be a matching primary key.

Rule 11: Distribution independence. A relational DBMS has distribution independence. It

means that if a distributed database than it should be possible to execute all operations on

it without being restricted by the physical locations of data.

Rule 12: Non-subversion rule. A low-level language (i.e. a language working on the

principle of a single record at a time), must in no way be used to bypass the integrity rules

and constraints imposed in the higher level (multiple records at a time).

sql ch 4

Documents

sql names

sql products

sql language

table names

sql basics ch

ansi sql standard

salesqualified column

data type store numbers