chapter 8: sql-99

Chapter 8: SQL-99Chapter 8: [email protected]

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ObjectivesObjectivesSQL standard

◦Data Definitions, Constraints, and Schema Changes in SQL2

◦Queries in SQL (basic and complex SQL Queries)

◦Update operations (delete, insert, and update statements)

◦Views (or Virtual Tables ) in SQL◦Specifying General Constraints as

Assertions


History of SQLHistory of SQLSQL stands for Structured Query

LanguageDeveloped by IBMadopted as standard language

for commercial RDBMS:◦ SQL-86 (or SQL-1) joint effort by ANSI and OSI◦ SQL-92 (or SQL2)◦ SQL-99( or SQL3)

CORE OPTIONAL


SQL (cont.)SQL (cont.)

A comprehensive non-procedural database language package that supports standard◦Supports both DDL and DML ◦Provides facilities to specify security,

authorization, and constraints


SQL Data Definition and Data SQL Data Definition and Data typestypesSQL uses

◦ Table (or relation)◦ Row (or tuple)◦ Column (or attribute)

Data Definition Commands◦ Create Schema ◦ Create tables ◦ Create Domain◦ Create view◦ Alter Table/Schema◦ Drop Table/Schema


SchemaSchemaSQL schema

◦Used to group tables and related constructs

◦identified by Schema Name Elements

tables constraints view, domains authorization constructs


Create SchemaCreate SchemaSchema is created using

◦CREATE SCHEMA◦E.g.,

CREATE SCHEMA Company AUTHORIZATION JSMITH

JSMITH is the Schema Owner ◦Catalog

Named collection of schemas Information_schema


Create Table Command in Create Table Command in SQLSQLCREATE TABLE

◦used to specify a new relation (or base table) CREATE TABLE EMPLOYEE CREATE TABLE COMPANY.EMPLOYEE

◦CREATE VIEW Used to create virtual tables

◦Attributes are ordered


Create table: exampleCreate table: example CREATE TABLE DEPARTMENT (

DNAME VARCHAR(10) NOT NULL,DNUMBER INTEGER NOT NULL,MGRSSN CHAR(9),MGRSTARTDATE CHAR(9) );


More exampleMore example Key attributes can be specified via the PRIMARY KEY and

UNIQUE phrasesCREATE TABLE DEPT (DNAME VARCHAR(10) NOT NULL,DNUMBER INTEGER NOT NULL,MGRSSN CHAR(9),MGRSTARTDATE CHAR(9),PRIMARY KEY (DNUMBER),UNIQUE (DNAME),FOREIGN KEY (MGRSSN) REFERENCES EMPLOYEE(SSN) );


SQL: Basic Data TypesSQL: Basic Data Types◦ Basic Data types

Numeric Integer, Real

Char string Fixed length (CHAR(n)) Varying length (VARCHAR(n)

bit-string, Fixed (BIT(n)) or varying VARYING(n)

date/time Boolean (T,F, Unknown) Timestamps (includes both date and time)


SQL: User Defined Data SQL: User Defined Data TypeType

◦User defined data type Domain in SQL

CREATE DOMAIN SSN_TYPE AS CHAR (9);


Specifying Constraints Specifying Constraints using SQLusing SQLConstraints and default values

can be specified on each ◦attributes◦tuple◦table


Specifying NULL and Default ValuesNULLS can be used as attribute

valuesA NOT NULL constraint can be

used to specify that NULL is not permitted

DEFAULT Value

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15


CHECK Clause

CHECK Clause ◦ Used to restrict attribute or domain values

E.g.,◦ To restrict department numbers between

integer 1-20 integer DNUMBER INT NOT NULL CHECK (DNUMBER>0 AND

DNUMBER <21)◦ Check clause can also be used to create the

domain CREATE DOMAIN D_NUM AS INTEGER CHECK (D_NUM>0

AND D_NUM <21) D_NUM can be used as attribute domain for

DNO Dnumber Dum


Specifying Key and Specifying Key and referential Integrity referential Integrity ConstraintsConstraints◦PRIMARY KEY CLUASE used to

specify PK E.g., PRIMARY KEY (DNUMBER); E.g., Dnumber INT PRIMARY KEY;

◦FOREIGN KEY CLUASE used to specify FK FOREIGN KEY (DNUMBER) REFERENCES

DEPARTMENT (DUNMBER)

◦UNIQUE CLAUSE Used for secondary keys

◦ Figure 8.1


Violation of Integrity Violation of Integrity Constraints (IC)Constraints (IC) Update/delete/insertion of tuples may violate

referential integrity constraints◦ The default action is Reject the operation

Schema Designer can specify an alternative action using referential triggered action clause to any FK constraint◦ Option include

SET DEFAULT SET NULL CASACADE

◦Option must be qualified ON DELETE or ON UPDATE


20

Referential IntegrityReferential Integrity

MINT

EMPLOEE

FNAME LNAME SSN BDATE ADDRESS SEX SUPERSSN DNO

DEPT

DEPT_LOC

SNAME DNUM MGRSSNMSDATE

DNUM DLOC

DNO should be Null or

should match a PK of DEPT


Named constraintsNamed constraintsNames to constraints

◦Used to identify/modify a particular constraint

◦Works like CONSTANT declaration in Program Languages

◦ Figure 8.2


23


Specifying Constraints on Specifying Constraints on Tuples Using CHECKTuples Using CHECK

◦Used at the end of schema◦Applies to individual tuples◦Use CHECK for more general

constraints E.g.,

CHECK (DEPT_CREATE_DATE < MGRSTARTDATE);


SCHEMA CHANGE SCHEMA CHANGE COMMANDS IN SQLCOMMANDS IN SQLTHE DROP COMMAND

◦CASCADE◦RESTRICT◦Used to drop the named schema

elements from database schema◦E.g.,

DROP SCHEMA COMPANY CASCADE DROP TABLE DEPENDENT CASCADE

◦RESTRCIT Used to drop the schema if it has no

elements in it◦Delete vs. Drop

Use delete if you want to delete the records but not the table definition


The ALTER TABLE The ALTER TABLE CommandCommandUsed to change the definition of

base table◦ E.g.,

ALTER TABLE COMPANY.EMPLOYEE ADD JOB VARCHAR(12)

ALTER TABLE COMPANY.EMPLOYEE DROP ADDRESS CASCADE;

ALTER TABLE COMPANY.DEPARTMENT ALTER MGRSSN DROP DEFAULT;

ALTER TABLE COMPANY.DEPARTMENT ALTER MGRSSN SET DEFAULT “344556677”

ALTER TABLE COMPANY.EMPLOYEE DROP CONSTRAINT EMPSUPERFK CASCADE


Basic Queries in SQLBasic Queries in SQLSQL has only one basic

statement to retrieve information ◦SELECT◦FROM◦WHERE

no relationship to the operation of relational algebra

Important features◦supports the notion of multi-set (or

Bag)


The Select-From-Where The Select-From-Where Structure of SQL QueriesStructure of SQL QueriesGeneral Form:

◦SELECT <attributes list>◦FROM <tables list>◦WHERE <condition>;


Some example: Query 0Some example: Query 0Get the birthday and address of

the employee(s) whose name is ‘John B. Smith’◦SELECT BDATE, ADDRESS◦FROM EMPLOYEE◦WHERE FNAME=‘John’ AND MINIT

=‘B’ AND LNAME = ‘SMITH’


Query 1Query 1Get the name and address of

all employee who work for the ‘Research’ Dept.◦SELECT FNAME, LNAME, ADDRESS ◦FROM EMPLOYEE, DEPARTMENT◦WHERE DNAME=‘Research’ AND

DNUMBER =DNO


Query 2Query 2

For every project located in ‘Stafford’, list the project number, the controlling department number, and the department manager’s last name, address, and birthrate◦ SELECT PNUMBER, DNUM, LNAME, ADDRESS, BDATE

◦ FROM PROJECT, DEPARTMENT, EMPLOYEE

◦ WHERE DNUM=DNUMBER AND MGRSSN=SSN

AND PLOCATION = ‘Stafford’


Ambiguous Attribute Names and Ambiguous Attribute Names and

RenamingRenaming In SQL, same name can be used

◦For more than one attribute in different tables

◦used in recursive queries


Example 1: Same name Example 1: Same name different tablesdifferent tablesTo remove ambiguity, we need to

qualify the attributes : use ‘.’ separator to qualify the attribute

e.g., suppose LNAME=NAME, and DNO=DNUMBER , DNAME=NAME SELECT FNAME, EMPLOYEE.NAME, ADDRESS FROM EMPLOYEE, DEPARTMENT WHERE DEPARTMENT.DNUMBER =

EMPLOYEE.DNUMBER AND DEPARTMENT.NAME = ‘Research


Example 2: Recursive Example 2: Recursive relationshipsrelationshipsFor each employee, find the

employee’s first and last name and the first and last name of her/his immediate supervisor◦ SELECT E.FNAME, E.LNAME, S.FNAME, S.LNAME◦ FROM EMPLOYEE AS E, EMPLOYEE AS S◦ WHERE E.SUPERSSN=S.SSN


Result of QueryE.Fname E.Lname S.Fname S.Lname

Jongm Smith Franklin Wong

Franklin Wong James Borg

Alicia Zelaya Jennifer Wallace

… … … …

.


SQL: Unspecified WHERE-SQL: Unspecified WHERE-ClauseClauseNo WHERE-clause means no

conditions No condition means Cross

product operations ()


Example Q10: No WHERE Example Q10: No WHERE ClauseClauseGet all combinations of

EMPLOYEE.SSN and DEPARTMENT.DNAME ◦SELECT SSN, DNAME◦FROM EMPLOYEE, DEPARTMENT

Get all Employee SSN◦SELECT SSN◦FROM EMPLOYEE


Example: Use of ‘*’ in Example: Use of ‘*’ in selectselectUse * to get all attributesGet all employees working for Dept. 5

◦SELECT *◦FROM EMPLOYEE◦WHERE DNO=5

E.g., 2◦SELECT *◦FROM EMPLOYEE, DEPARTMENT;◦WHERE DNO=5 and Dname=‘Research’


Tables as Set in SQLTables as Set in SQL SQL treats tables as a multi-set (i.e., a set

having duplicates) Why?

◦ Duplicate elimination is an expensive operation (sort and delete)

◦ user may be interested in the result of a query◦ in case of aggregate function, we do not want to

eliminate duplicatesSQL Table with a key is a SET by definition

◦ Why? Because key must be unique To treat tables as sets use DISTINCT in

SELECT statement


Query 11 (Query 11 (ALLALL))◦Retrieve the salary of every employee,

SELECT ALL SALARY FROM EMPLOYEE

41

Salary

3000

4000

2500

2500

…

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Query 12 (Query 12 (DISTINCTDISTINCT))Get the salary of every employee

USING distinct (set)◦SELECT DISTINCT SALARY◦FROM EMPLOYEE

42

Salary

3000

4000

2500

4300

5500

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SQL and NULLSQL and NULL

NULL represents◦ Value is not known

e.g., an unknown address◦ Value is not available

E.g., unlisted phone number◦ Value does not apply

E.g., Unmarried employee has no name for his/her spouse


NULL and Comparison Operators Comparisons involving NULL

◦ When NULL is involved in comparison, then the result considered to be unknown

◦ Unknown (maybe true or maybe false) SQL uses 3-valued logic

◦ TRUE,◦ FALSE, ◦ UNKNOWN

How define the SQL evaluate the 3-valued logical expressions involving ◦ AND◦ OR ◦ NOT


Truth table for 3-valued logicTruth table for 3-valued logicx y x AND y x OR y Not x

TRUE TRUE TRUE TRUE FALSE

TRUE UNKNOWN UNKNOWN TRUE FALSE

TRUE FALSE FALSE TRUE FALSE

UNKNOWN TRUE UNKNOWN TRUE UNKNOWN

UNKNOWN UNKNOWN UNKNOWN UNKNOWN UNKNOWN

UNKNOWN FALSE FALSE UNKNOWN UNKNOWN

FALSE TRUE FALSE TRUE TRUE

FALSE UNKNOWN FALSE UNKNOWN TRUE

FALSE FALSE FALSE FALSE TRUE


More on 3-valueIn SELECT-PROJECT-JOIN queries,

only those combinations of tuples that are evaluated to TRUE are selected


Query involved NULLQuery involved NULLQ18: Get the names of all

employees who do not have supervisors◦SELECT Fname, Lname◦FROM EMPLOYEE◦WHER Super_ssn IS NULL


Query using Query using Union, Union, Intersection, EXECEPTIntersection, EXECEPTComplex SQL queries can be

formulated using ◦UNION◦INTERSECTION◦EXECEPT


Example of UNION: Q4AExample of UNION: Q4AMake a list of Project numbers for

projects that involve an employee whose last name is ‘Smith’, either as a worker or as a manger of the department that controls the project


Query 4: Using Union Query 4: Using Union QueryQuery

◦ (SELECT DISTINCT PNUMBER ◦ FROM PROJECT, DEPARTMENT, EMPLOYEE WHERE DNUM=DNUMBER AND MGRSSN=SSN AND LNAME=‘Smith’)◦ UNION◦ (SELECT DISTINCT PNUMBER ◦ FROM WORKS_ON, EMPLOYEE , PROJECT◦ WHERE PNUMBR = PNO AND ESSN=SSN AND

LNAME=‘Smith’)


Complex SQL Queries: Complex SQL Queries: Nested QueriesNested QueriesSome queries need the existing

values in the database to be retrieved & compared

Nested queries ◦used to formulate such queries


Q4: Using Nested queryQ4: Using Nested queryMake a list of Project numbers for

projects that involve an employee whose last name is ‘Smith’, either as a worker or as a manger of the department that controls the project


Query Q4A: Using nested Query Q4A: Using nested queryquery

SELECT DISTINCT PNUMBER FROM PROJECT

WHERE PNUMBER IN (SELECT PNUMBER

FROM DEPARTMENT, EMPLOYEE , PROJECT WHERE DNUM = DNUMBE AND MGRSSN = SSN AND LNAME =

‘Smith’) OR

PNUMBER IN ( SELECT PNO

FROM WORKS_ON, EMPLOYEE WHERE ESSN=SSN AND LNAME = ‘Smith’);


Correlated nested queriesCorrelated nested queriesCorrelated Queries:

◦When a condition in WHERE clause of a nested query references attribute(s) of a relation defined in the outer query


Query 16:Example of Query 16:Example of Correlated queryCorrelated query Get the name of each employee who has a

dependent with the same first name and the same sex as the employee

SELECT E.FNAME, E.LNAME FROM EMPLOYEE AS E WHERE E.SSN IN (SELECT ESSN

FROM DEPENDENT

WHERE E.FNAME=DEPENDENT_NAME

AND E.SEX=SEX);


Q16B: Alternative QueryQ16B: Alternative QueryA nested query involving ‘=‘ or ‘IN’ can be

replaced by simple query◦ SELECT E.FNAME,E.LNAME◦ FROM EMPLOYEE AS E, DEPEDENT AS D◦ WHERE E.SSN=D.ESSN AND E.FNAME=D.DEPENDENT_NAME

AND E.SEX=D.SEX;


More on Set comparisonsMore on Set comparisonsGet SSN of all employees who

work on the same combination (project, hours) on some project that employee ‘John Smith’ with SSN=‘123456789’◦ SELECT DISTINCT ESSN◦ FROM WORKS_ON◦ WHERE (PNO, HOURS) IN (SELECT PNO, HOURS FROM WORKS_ON WHERE ESSN=‘123456789’)


THE EXISTS AND UNIQUE THE EXISTS AND UNIQUE FUNCTIONS IN SQLFUNCTIONS IN SQLEXISTS:

◦Checks the result (i.e, SET) of the correlated nested query to see if it is empty or not If the result (i.e., set) is non-empty it

returns TRUE If the result is empty it returns FALSE


Query 16: example of Query 16: example of EXISTS EXISTS Find the name of each employee

who has a dependent with the same first name and same sex as the employee◦SELECT E.FNAME, E.LNAME◦FROM EMPLOYEE AS E◦WHERE EXISTS ( SELECT * ◦ FROM DEPENDENT ◦ WHERE E.SSN=ESSN

AND SEX=E.SEX AND E.FNAME=DEPENT_NAME)


Query 16: example of NOT Query 16: example of NOT EXISTS EXISTS Find the name of each employee

who has no dependent ◦ SELECT FNAME, LNAME◦ FROM EMPLOYEE E◦ WHERE NOT EXISTS ◦ (SELECT * ◦ FROM DEPENDENT ◦ WHERE SSN=ESSN)


EXPLICIT SETS AND NULLS EXPLICIT SETS AND NULLS IN SQLIN SQLRetrieve SSNs of all employees

who work on project number 12,22,or 33◦SELECT DISTINCT ESSN◦FROM WORKS_ON◦WHERE PNO IN (12,22,33)


Join (revisited): FROM Join (revisited): FROM CLAUSECLAUSEJoin operation can be specified in

FROM clause◦Understandability◦Use AS construct to Rename join

attributes◦Default is inner join


Example 1: Join in FORMExample 1: Join in FORMFind the name and address of

employees who work in Research department◦SELECT FNAME, LNAME, ADDRESS◦FROM (EMPLOYEE JOIN DEPARTMENT

ON DNO=DNUMBER)

◦WHERE DNAME = ‘Research’


Example 2: Renaming and Example 2: Renaming and NATURAL JOINNATURAL JOIN

◦SELECT FNAME, LNAME, ADDRESS◦FROM ( EMPLOYEE NATURAL JOIN

(DEPARTMENT AS DEPT (DNAME, DNO, MSSN, MSDATE)))

◦WHERE DNAME =‘Research’


The keyword ALLThe keyword ALLALL is comparison operatorsCan be used with any other

comparison operators◦ E.g.

v> ALL V where V is a SET It returns TRUE if v > every element in V


Example KEYWORD ALLExample KEYWORD ALLget the names of all employees

whose salary is greater than the salary of ALL the employees in department 5◦SELECT LNAME, FNAME◦FROM EMPLOYEE◦WHERE SALARY > ALL

(SELECT SALARY FROM EMPLOYEE WHERE DNO=5);


SCOPE of nested queriesSCOPE of nested queriesE.g.,

◦SELECT E.FNAME, E.LNAME◦FROM EMPLOYEE AS E◦WHERE E.SSN ◦IN (SELECT ESSN FROM DEPENDENT ◦ WHERE ESSN=E.SSN AND

E.FNAM=DEPENDENT_NAME AND SEX=E.SEX


Aggregate FunctionsAggregate FunctionsQ20: Find the sum of the salaries of all employees

in the ‘Research’ dept, as well as the max salary, the min salary, and average in that dept.

◦SELECT SUM(SALARY), MAX(SALARY), MIN(SALARY) AVG(SALARY)

◦FROM (EMPLOYEE NATURAL JOIN (DEPARTMENT AS DEPT(Dname, Dno, Mssn, Msdate)))

◦WHERE DNAME=‘Research’


Example of grouping and Example of grouping and COUNTCOUNT GROUP BY CLUASE

◦ Used to partition the relation into sub-relation◦ Works with aggregate functions (e.g. COUNT)

grouping attribute Grouping attribute (s) need to be specified in SELECT clause Create separate group for the grouping attribute with NULL

values


QUERY 24 (GROUP BY)QUERY 24 (GROUP BY)For each department, retrieve the

department number, the number of employees in the department, and their average salary◦SELECT DNO, COUNT(*), AVG(SALARY)

◦FROM EMPLOYEE◦GROUP BY DNO; Figure.8.6.(a)


Figure 8.4Figure 8.4


HAVING ClauseHAVING ClauseUsed with GROUP BY clauseUsed as a condition on the sub-

relations or groups◦ Groups satisfying the conditions are selected


Query 26Query 26For each project on which more than two

employees work, get the project number, the project name, the number of employees who work on the project◦SELECT PNUMBER, PNAME, COUNT(*)◦FROM PROJECT, WORKS_ON◦WHERE PNUMBER=PNO◦GROUP BY PNAME, PNUMBER◦HAVING COUNT(*)>2;◦Fig.8.6.(b) and (c)


Figure 8.4Figure 8.4


Q28: COUNT and HAVING Q28: COUNT and HAVING Clauses (revisited)Clauses (revisited)Count the total number of

employees whose salaries exceed $40,000 in each department, but only for department where more than five employees work


Version 1: ?Version 1: ?SELECT DNAME, COUNT (*)FROM DEPARTMENT, EMPLOYEEWHERE DNUMBER=DNO AND

SALARY>40000GROUP BY DNAMEHAVING COUNT (*)>5;Wrong

◦ Selects only department having more than 5 employees who ALL make more than 40K


Version 2: Correct OneVersion 2: Correct OneSELECT DNUMBER, COUNT (*)FROM DEPARTMENT, EMPLOYEEWHERE DNUMBER=DNO AND

SALARY>4000 AND DNO IN ◦(SELECT DNO ◦ FROM EMPLOYEE ◦ GROUP BY DNO ◦ HAVING COUNT(*)>5)

GROUP BY DNUMBER


Precedence rule Precedence rule The processing steps

◦1) the WHERE Clause is executed to select individual tuples

◦2) HAVING clause executed to select individual groups of tuples


SQL: Insert, Delete, and SQL: Insert, Delete, and Update commandsUpdate commandsThe Insert Command

◦To add a new tuple to employee INSERT INTO EMPLOYEE VALUES

(‘Richard’,’K’,’Marini’,653298653,’30-dec-52’, ‘98 Oak Forest, Katy, TX’,’M’, 37000,’987654321’, 4)


More on InsertMore on InsertTo enter a new tuple using

known attributes◦INSERT INTO EMPLOYEE (FNAME,

LNAME, SSN)◦VALUES (‘Richard’, ’Marini’,

‘653298653’); the tuple is accepted, if no constraint is

violated,


Insert Multiple tuplesInsert Multiple tuples INSERT command can be used to load

multiple tuples as the same time◦ Suppose

CREAT TABLE DEPT_INFO (DEPT_NAME VARCHAR(15), NO_OF_EMPS INTEGER, TOTAL_SAL INTEGER);

INSERT INTO DEPTS_INFO (DEPT_NAME, NO_OF_EMPS,TOTAL_SAL)

SELECT DNAME, COUNT(*), SUM (SALARY)FROM (DEPARTMENT JOIN EMPLOYEE ON

DNUMBER=DNO)GROUP BY DNAME;


The DELECT CommandThe DELECT CommandRemoves tuples from a relation

one at a time◦DELETE FROM EMPLOYEE◦WHERE LNAME=‘Brown’


Multiple DELETESMultiple DELETESOne or more tuples can be

deleted◦E.g.. Delete all employees who work

for “Research” dept. DELETE FROM EMPLOYEE WHERE DNO IN

(SELECT DNUMBER FROM DEPARTMENT WHERE DNAME=‘Research’);


More on DELETEMore on DELETEDELETE FROM EMPLOYEE

◦Creates empty table


The UPDATE CommandThe UPDATE Command

Used to modify values of one (or multiple) selected tuples associated with ONE relation

E.g.◦ Change the location and controlling

department number of project number 10 to ‘Grand Forks’ and 5

◦ UPDATE PROJECT◦ SET PLOCATION = ‘Grand Forks’, DNUM=5◦ WHERE PNUMBER=10;


Example: Multiple Example: Multiple changeschangesE.g.,

◦ Give all employees working in ‘Research’ department a 10 percent salary raise UPDATE EMPLOYEE SET SALARY = SALARY *1.1 WHERE DNO IN ( SELECT DNUMBER

FROM DEPARTMENT WHERE DNAME = ‘Research’);


Additional featuresAdditional featuresSpecifying ASSERTIONS/TriggersCreating ViewsWriting programsSpecifying physical DBGranting and revoking privilegesSpecifying Complex ObjectsConcurrency Control/Recovery

mechanisms


Specifying General Specifying General ConstraintsConstraintsgeneral constraints can be

specified as follows◦ CREATE ASSETION SALARY_CONSTRAINT

CHECK ( NOT EXISTS ( SELECT * FROM EMPLOYEE AS E,

EMPLOYEE AS M, DEPARTMENTAS D WHERE E.SALARY > M.

SALARY AND E.DNO=D.NUMBER

AND D.MGRSSN=M.SSN));


Constraints vs. TriggersConstraints vs. TriggersConstraints

◦Prevent the data from being made inconsistent by any kind of statement

Triggers◦Maintain database consistency and

defined operationally


Views in SQLViews in SQL A view refers to a single table that is derived

from other tables◦ CREAT VIEW WORKS_ON1◦ AS SELECT FNAME, LNAME, PNAME, HOURS

FROM EMPLOYEE, PROJECT, WORKS_ON WHERE SSN=ESSN AND PNO=PNUMBER

90

FNAME LNAME PNAME HOURS

WORKS_ON1


Example 2Example 2◦ CREAT VIEW DEPT_INFO(DEPT_NAME,

NO_OF_EMPS, TOTAL_SAL)◦ AS SELECT DNAME, COUNT(*), SUM(SALARY)

FROM EMPLOYEE, DEPARTMENT WHERE DNUMBER=DNO GROUP BY DNAME;

91

DEPT_NAME TOTAL_SALNO_OF_EMPS

DEPT_INFO


Query on ViewQuery on ViewSELECT Fname, LnameFROM WORKS_ON1WHERE Pname =‘ProjectX’;


DROP VIEWDROP VIEWDROP VIEW WORKS_ON1;


Database Views: Database Views: applications applications Used as subroutine mechanism

◦make a complex query easier to understand/reuse

◦easier to debugused as a flexible mechanism for

access CONTROL to the data (security)


View PropertiesView PropertiesA View is always up to dateA view is realized at the time we

execute a query


View ImplementationView ImplementationTwo strategies to implement a

view◦Query modification

Maps the view into the base tables

◦View materialization Create a temporary view table Uses incremental approach to keep a

materialized table up-date Removes the view table if it is not

accessed for certain period of time


Query ModificationQuery ModificationQuery on

◦SELECT Fname, Lname◦FROM WORKS_ON1◦WHERE Pname =‘ProjectX’;

Translated to◦ SELECT Fname, Lname,

FROM EMPLOYEE, PROJECT, WORKS_ON WHERE SSN=ESSN AND PNO=PNUMBER AND Pname =‘ProjectX’;


Updating of ViewsUpdating of ViewsUpdating the views can be

complicated and ambiguousan update on a view defined on a

single table without any aggregate functions can be mapped to an update on the base table


Table: 8.1 (SQL Summary)Table: 8.1 (SQL Summary)


chapter 8: sql-99

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