DatabaseIM ISU1 Fundamentals of Database Systems Chapter 8 SQL99 - The Relational Database Standard.

IM ISU 1Database

Fundamentals of

Database Systems

Chapter 8

SQL99 - The Relational Database Standard

IM ISU 2Database

Data Definition in SQL Schema and Catalog Concepts in SQL2

An SQL schema is identified by » A schema name» An authorization identifier to indicate the user or

account who owns the schema » Descriptors for each element in the schema

Schema elements include the tables, constraints, views, domains, and other constructs (such as authorization grants) that describe the schema

IM ISU 3Database

Data Definition in SQL (cont.) A schema is created via the CREATE

SCHEMA statement» Example

Create a schema called COMPANY, owned by the user with authorization identifier JSMITH:

CREATE SCHEMA COMPANY AUTHORIZATION JSMITH;

IM ISU 4Database

Data Definition in SQL (cont.) Catalog

» A named collection of schemas in an SQL environment

» A catalog always contains a special schema called INFORMATION_SCHEMA, which provides information on all the element descriptors of all the schemas in the catalog to authorized users

IM ISU 5Database

Data Definition in SQL (cont.) Purpose

Used to CREATE, DROP, and ALTER the descriptions of the tables (relations) of a database

CREATE TABLE Specifies a new base relation by giving it a

name, and specifying each of its attributes and their data types

IM ISU 6Database

Data Definition in SQL (cont.) Data types in SQL2

» Numeric– INT or INTEGER– SMALLINT– FLOAT– REAL– DOUBLE PRECISION– DEC(i, j) or DECIMAL(i,j) or NUMERIC(i,j)

» Character-string– CHAR(n) or CHARACTER(n)

– VARCHAR(n) or CHAR VARYING(n) or CHARACTER VARYING(n)

IM ISU 7Database

Data Definition in SQL (cont.)» Bit-string

– BIT(n)

– BIT VARYING(n)

» Date – In the form YYYY-MM-DD

» Time – In the form HH:MM:SS

» TIME(i)– Made up of hour:minute:second plus i additional digits specifying fractions of a second– format is hh:mm:ss:ii...i

IM ISU 8Database

Data Definition in SQL (cont.)» TIMESTAMP

– Has both DATE and TIME components

» INTERVAL– Specifies a relative value rather than an absolute

value

– Can be DAY/TIME or YEAR/MONTH intervals

– Can be positive or negative

– when added to or subtracted from an absolute value, the result is an absolute value

IM ISU 9Database

Data Definition in SQL (cont.) In SQL2, it is possible to specify the domain

of each attribute and declare the domain name, e.g.,

CREATE DOMAIN SSN_TYPE AS CHAR(9);

» We can use SSN_TYPE in place of CHAR(9)

IM ISU 10Database

Data Definition in SQL (cont.) A constraint NOT NULL may be specified on an attribute

CREATE TABLE DEPARTMENT( DNAME VARCHAR(10) NOT NULL,

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

In SQL2, the CREATE TABLE can be used to specify the primary key, secondary keys, and referential integrity constraints (foreign keys)

IM ISU 11Database

Data Definition in SQL (cont.)» Primary key attributes can be specified via the

PRIMARY KEY » Key attributes can be specified via UNIQUE» Referential integrity is specified via the

FOREIGN KEY clauseCREATE 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 EMP);

IM ISU 12Database

Data Definition in SQL (cont.) It is also possible to define a default value for

an attribute by appending the clause DEFAULT <value> to an attribute definition

IM ISU 13Database

Data Definition in SQL (cont.)

IM ISU 14Database

Data Definition in SQL (cont.) In SQL2, action can be specified if a referenti

al integrity constraint is violated upon deletion of a referenced tuple or upon modification of a referenced primary key » Attaching a referential triggered action clause to

any foreign key constraint » The options include SET NULL, CASCADE, an

d SET DEFAULT» An option must be qualified with either ON DEL

ETE or ON UPDATE

IM ISU 15Database

Data Definition in SQL (cont.)» SET NULL (DEFAULT) ON DELETE: the valu

e of the deleted referencing attributes is changed to NULL or the specified default value

» SET NULL (DEFAULT) ON UPDATE: the value of the updated referencing attributes is changed to NULL or the specified default value

» CASCADE ON DELETE: delete all the referencing tuples

» CASCADE ON UPDATE: change the value of the foreign key to the updated (new) primary key value for all referencing tuples

IM ISU 16Database

Data Definition in SQL (cont.)» A constraint may be given a name, following the

keyword CONSTRAINT» The names of all constraints within a particular

schema must be unique

IM ISU 17Database

Data Definition in SQL (cont.) Base tables (or base relations)

» The relations declared through CREATE TABLE statements are called

» The relation and its tuples are actually created and stored as a file by the DBMS

Virtual relations» The relations created through the CREATE VIE

W statement » The relations may or may not correspond to an ac

tual physical file

IM ISU 18Database

Data Definition in SQL (cont.) DROP SCHEMA

Used to a whole schema There are two drop behavior options

» CASCADE: remove the database schema and all its tables, domains, and other elements, e.g.,

DROP SCHEMA COMPANY CASCADE; » RESTRICT: the schema is dropped only if it has

no elements

IM ISU 19Database

Data Definition in SQL (cont.) DROP TABLE

Used to remove a relation (base table) and its definition

The relation can no longer be used in queries, updates, or any other commands since its

description no longer exists

DROP TABLE DEPENDENT;

IM ISU 20Database

Data Definition in SQL (cont.) Two options

» CASCADE: all constraints and views that reference the table are dropped automatically along with the table itself

DROP TABLE DEPENDENT CASCADE;

» RESTRICT: a table is dropped only if it is not referenced in any constraints or views

IM ISU 21Database

Data Definition in SQL (cont.) ALTER TABLE

Used to change the base relation definition Possible actions include

» adding or dropping a column (attribute) » changing a column definition» adding or dropping table constraints

ExampleALTER TABLE COMPANY.EMPLOYEE ADD JOB VARCHAR(12);

IM ISU 22Database

Characteristics of Relations

» The new attribute will have NULLs in all the tuples of the relation» Hence, the NOT NULL constraint is not allowed for such an attribute

» The database users must still enter a value for the new attribute JOB for each EMPLOYEE tuple

» This can be done using the UPDATE command

IM ISU 23Database


To drop a column, we must choose either CASCADE or RESTRICT for drop behavior» CASCADE: all constraints and views that

reference the column are dropped automatically along with the column

» RESTRICT: the column is drop only if no views or constraints reference the column

» Example

ALTER TABLE COMPANY.EMPLOYEE DROP ADDRESS CASCADE;

IM ISU 24Database


It is also possible to alter a column definition by dropping an existing default clause or by defining a new default clause» Example 1

ALTER TABLE COMPANY.DEPARTMENT ALTER MGRSSN DROP DEFAULT;

» Example 2

ALTER TABLE COMPANY.DEPARTMENT ALTER MGRSSN SET DEFAULT "333445555";

IM ISU 25Database

Retrieval Queries in SQL

Preliminary SQL has one basic statement for retrieving

information from a database; the SELECT statement

This is not the same as the SELECT operation of the relational algebra

IM ISU 26Database

Retrieval Queries in SQL (cont.)

Important distinction between SQL and the formal relational model » SQL allows a table (relation) to have two or more

identical tuples» Hence, an SQL relation (table) is a multi-set (so

metimes called a bag) of tuples; it is not a set of tuples

» SQL relations can be constrained to be sets by specifying PRIMARY KEY or UNIQUE attributes, or by using the DISTINCT option in a query

IM ISU 27Database


Basic form of the SQL SELECT statementSELECT <attribute list>FROM <table list>WHERE <condition>» <attribute list> is a list of attribute names whose

values are to be retrieved by the query» <table list> is a list of the relation names required

to process the query» <condition> is a conditional (Boolean) expressio

n that identifies the tuples to be retrieved by the query

IM ISU 28Database


Simple SQL Queries Basic SQL queries correspond to using the

SELECT, PROJECT, and JOIN operations of the relational algebra

All subsequent examples use the COMPANY database

Example of a simple query on one relation

IM ISU 29Database


IM ISU 30Database


IM ISU 31Database

Retrieval Queries in SQL (cont.) Query 0: Retrieve the birthdate and address of t

he employee whose name is 'John B. Smith'Q0: SELECT BDATE, ADDRESS

FROM EMPLOYEEWHERE FNAME='John' AND

MINIT='B' AND LNAME='Smith’;

» Similar to a SELECT-PROJECT pair of relational algebra operations

– the SELECT-clause specifies the projection attributes – the WHERE-clause specifies the selection condition

IM ISU 32Database


BDATE,ADDRESS (FNAME=‘John’ AND MINIT=‘B’ AND LNAME=‘Sm

ith’ (EMPLOYEE))

» However, the result of the query may contain duplicate tuples

» Result of Q0

BDATE ADDRESS

1965-01-09 731 Fondren, Houston, TX

IM ISU 33Database

Retrieval Queries in SQL (cont.) Query 1: Retrieve the name and address of all

employees who work for the 'Research' departmentQ1: SELECT FNAME, LNAME,

ADDRESSFROM EMPLOYEE,

DEPARTMENTWHERE DNAME='Research' AND

DNUMBER=DNO;» Similar to a SELECT-PROJECT-JOIN sequence of

relational algebra operations– (DNAME='Research') is a selection condition– (DNUMBER=DNO) is a join condition

IM ISU 34Database

Retrieval Queries in SQL (cont.) Query 2: For every project located in 'Stafford', list

the project number, the controlling department number, and the department manager's last name, address, and birthdate.Q2: SELECT PNUMBER, DNUM, LNAM

E,BDATE, ADDRESS

FROM PROJECT, DEPARTMENT, EMPLOYEE

WHERE DNUM=DNUMBER AND MGRSSN=SSN ANDPLOCATION='Stafford’;

IM ISU 35Database

Retrieval Queries in SQL (cont.)» In Q2, there are two join conditions» The join condition DNUM=DNUMBER relates a

project to its controlling department» The join condition MGRSSN=SSN relates the

controlling department to the employee who manages that department

IM ISU 36Database

Retrieval Queries in SQL (cont.) Aliases (Renaming) with SQL

In SQL, we can use the same name for two (or more) attributes as long as the attributes are in different relations

A query that refers to two or more attributes with the same name must qualify the attribute name with the relation name by prefixing the relation name to the attribute name» e.g., EMPLOYEE.LNAME, DEPARTMENT.DNAME

IM ISU 37Database

Retrieval Queries in SQL (cont.) Some queries need to refer to the same relation

twice; In this case, aliases are given to the relation name

Query 8: For each employee, retrieve the employee's name, and the name of his or her

immediate supervisorQ8: SELECT E.FNAME, E.LNAME,

S.FNAME, S.LNAMEFROM EMPLOYEE E SWHERE E.SUPERSSN=S.SSN;

IM ISU 38Database

Retrieval Queries in SQL (cont.)» In Q8, the alternate relation names E and S are

called aliases for the EMPLOYEE relation» We can think of E and S as two different copies of

the EMPLOYEE relation– E represents employees in the role of supervisees

– S represents employees in the role of supervisors

» We an also use the AS keyword to specify aliases, e.g., for Q8

FROM EMPLOYEE E S

FROM EMPLOYEE AS E, EMPLOYEE AS S

IM ISU 39Database

Retrieval Queries in SQL (cont.)» Aliasing can also be used in any SQL query for

convenience

Q1B: SELECT E.FNAME, E.NAME, E.ADDRESS

FROM EMPLOYEE E, DEPARTMENT D

WHERE D.NAME=‘Research’ AND

D.DNUMBER=E.DNUMBER;

IM ISU 40Database

Retrieval Queries in SQL (cont.)Unspecified WHERE-clause

A missing WHERE-clause indicates no condition; hence, all tuples of the relations in the FROM-clause are selected

This is equivalent to WHERE TRUE Query 9: Retrieve the SSN values for all empl

oyeesQ9: SELECT SSN

FROM EMPLOYEE;

IM ISU 41Database

Retrieval Queries in SQL (cont.) If more than one relation is specified in the FRO

M-clause and there is no join condition, then the CARTESIAN PRODUCT of tuples is selected» Query 10: Retrieve all combinations of EMPLOYEE S

SN and DEPARTMENT DNAME

Q10: SELECT SSN, DNAME

FROM EMPLOYEE, DEPARTMENT;

» It is extremely important not to overlook specifying any selection and join conditions in the WHERE-clause

IM ISU 42Database

Retrieval Queries in SQL (cont.)Use of *

To retrieve all the attribute values of the selected tuples, a * is used, which stands for all the attributes

Query 1C: Retrieves all the attribute values of EMPLOYEE at DEPARTMENT number 5 Q1C: SELECT *

FROM EMPLOYEE

WHERE DNO=5;

IM ISU 43Database

Retrieval Queries in SQL (cont.) Query Q1D: Retrieves all the attributes of an

EMPLOYEE and the attributes of the DEPARTMENT he or she works in, for every employee of the ‘Research’ departmentQ1D: SELECT *

FROM EMPLOYEE, DEPARTMENT

WHERE DNAME='Research' AND

DNO=DNUMBER;

IM ISU 44Database

Retrieval Queries in SQL (cont.) Use of DISTINCT

SQL does not treat a relation as a set; duplicate tuples can appear

To eliminate duplicate tuples in a query result, the keyword DISTINCT is usedQ11: SELECT SALARY

FROM EMPLOYEE;

Q11A: SELECT DISTINCT SALARY

FROM EMPLOYEE;

IM ISU 45Database

Retrieval Queries in SQL (cont.) » The result of Q11 may have duplicate SALARY

values whereas Q11A does not have any duplicate values

IM ISU 46Database

Retrieval Queries in SQL (cont.) Set Operations

SQL has directly incorporated some set operations

There is a union operation (UNION), and in some versions of SQL there are set difference (MINUS) and intersection (INTERSECT) operations

The resulting relations of these set operations are sets of tuples; duplicates are eliminated

IM ISU 47Database

Retrieval Queries in SQL (cont.) The set operations apply only to union compatible relations

Query 4: Make a list of all project numbers for projects that involve an employee whose last name is 'Smith' as a worker or as a manager of the department that controls the project

IM ISU 48Database

Retrieval Queries in SQL (cont.) Q4: (SELECT PNAME

FROM PROJECT, DEPARTMENT, EMPLOYEE

WHERE DNUM=DNUMBER AND MGRSSN=SSN AND LNAME='Smith');

UNION

(SELECT PNAME

FROM PROJECT, WORKS_ON, EMPLOYEE

WHERE PNUMBER=PNO AND ESSN=SSNAND LNAME='Smith');

IM ISU 49Database

Retrieval Queries in SQL (cont.) Substring Comparison

The LIKE comparison operator is used to compare partial strings

Two reserved characters are used » '%' (or '*' in some implementations) replaces an

arbitrary number of characters» '_' replaces a single arbitrary character

Query 12: Retrieve all employees whose address is in Houston, Texas

IM ISU 50Database

Retrieval Queries in SQL (cont.) Q12: SELECT FNAME, LNAME

FROM EMPLOYEE

WHERE ADDRESS LIKE '%Houston,TX%’;

Query 12A: Retrieve all employees who were born during the 1950sQ12A: SELECT FNAME, LNAME

FROM EMPLOYEE

WHERE BDATE LIKE '__5_______’;

» Here, '5' must be the 3th character of the string

IM ISU 51Database

Retrieval Queries in SQL (cont.) Arithmetic Operations

The standard arithmetic operators '+', '-'. '*', and '/' can be applied to numeric values in an SQL query result

Query 13: Show the effect of giving all employees who work on the 'ProductX' project a 10% raise

Q13: SELECT FNAME, LNAME, 1.1*SALARYFROM EMPLOYEE, WORKS_ON, PROJECTWHERE SSN=ESSN AND PNO=PNUMBER

AND PNAME='ProductX’;

IM ISU 52Database

Retrieval Queries in SQL (cont.) ORDER BY

The ORDER BY clause is used to sort the tuples in a query result based on the values ofsome attribute(s)

The default order is in ascending order Query 15: Retrieve a list of employees and

the projects each works in, ordered by the employee's department, and within each department ordered alphabetically by employee last name

IM ISU 53Database

Retrieval Queries in SQL (cont.) Q15:SELECT DNAME, LNAME, FNAME,

PNAME FROM DEPARTMENT, EMPLOYEE,

WORKS_ON, PROJECTWHERE DNUMBER=DNO AND SSN=ESSN

AND PNO=PNUMBERORDER BY DNAME, LNAME, FNAME;

We can specify the keyword DESC if we want a descending orderORDER BY DNAME DESC, LNAME ASC,

FNAME ASC

IM ISU 54Database

More Complex Queries Nesting of Queries

A complete SELECT query, called a nested query , can be specified within the WHERE-clause of another query, called the outer query

Many of the previous queries can be specified in an alternative form using nesting

Query 1: Retrieve the name and address of all employees who work for the 'Research' department

IM ISU 55Database

More Complex Queries (cont.) Q1: SELECT FNAME, LNAME, ADDRESS

FROM EMPLOYEE

WHERE DNO IN

(SELECT DNUMBER

FROM DEPARTMENT

WHERE DNAME='Research');» The nested query selects the number of the 'Rese

arch' department» The outer query select an EMPLOYEE tuple if it

s DNO value is in the result of either nested query

IM ISU 56Database

More Complex Queries (cont.) » The comparison operator IN compares a value v

with a set (or multi-set) of values V, and evaluates to TRUE if v is one of the elements in V

» In general, we can have several levels of nested queries

» In this example, the nested query is not correlated with the outer query

A reference to an unqualified attribute refersto the relation declared in the innermostnested query

IM ISU 57Database

More Complex Queries (cont.) Query 16: Retrieve the name of each employee who has a dependent with the same first name and same sex as the employee

Q16: 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);

IM ISU 58Database

More Complex Queries (cont.) Correlated Nested Queries

If a condition in the WHERE-clause of a nested query references an attribute of a relation declared in the outer query, the two queries are said to be correlated

The result of a correlated nested query is different for each tuple (or combination of tuples) of the relation(s) the outer query

IM ISU 59Database

More Complex Queries (cont.) A query written with nested SELECT... FROM... WHERE... blocks and using the = or IN comparison operators can always be expressed as a single block query

For example, Q16 may be written as in Q16A

Q16A: SELECT E.FNAME, E.LNAME

FROM EMPLOYEE AS E, DEPENDENT AS D

WHERE E.SSN = D.ESSN AND E.SEX = D.SEX

AND E.FNAME=D.DEPENDENT_NAME;

IM ISU 60Database

More Complex Queries (cont.) The EXISTS Function

EXISTS is used to check whether the result of a correlated nested query is empty or not

We can formulate Query 16 in an alternative form that uses EXISTS as Q16B below

IM ISU 61Database

More Complex Queries (cont.) Q16: SELECT E.FNAME, E.LNAME

FROM EMPLOYEE AS E WHERE EXISTS

(SELECT * FROM DEPENDENT WHERE E.SSN = ESSN AND

E.SEX = SEX AND E.FNAME = DEPENDENT_NAME);

IM ISU 62Database

More Complex Queries (cont.) Query 6: Retrieve the names of employees who have no dependentsQ6: SELECT FNAME, LNAME

FROM EMPLOYEE

WHERE NOT EXISTS (SELECT *

FROM DEPENDENT

WHERE SSN=ESSN);

» In Q6, the correlated nested query retrieves all DEPENDENT tuples related to an EMPLOYEE

» If none exist, EMPLOYEE tuple is selected

IM ISU 63Database

More Complex Queries (cont.) Explicit Sets

It is also possible to use an explicit (enumerated) set of values in the WHERE-clause rather than a nested query

Query 17: Retrieve the social security numbers of all employees who work on project number 1, 2, or 3Q13: SELECT DISTINCT ESSN

FROM WORKS_ONWHERE PNO IN (1, 2, 3);

IM ISU 64Database

More Complex Queries (cont.) NULLS in SQL Queries

SQL allows queries that check if a value is NULL (missing or undefined or not applicable)

Any comparison with null returns unknown» E.g.

5 < null or null <> null or null = null

IM ISU 65Database

More Complex Queries (cont.) Three-valued logic using the truth value

unknown: OR: (unknown or true) = true, (unknown or false) =

unknown, (unknown or unknown) = unknown AND: (true and unknown) = unknown, (false and

unknown) = false, (unknown and unknown) = unknown

NOT: (not unknown) = unknown “P is unknown” evaluates to true if predicate P

evaluates to unknown

IM ISU 66Database

More Complex Queries (cont.) SQL uses IS or IS NOT to compare NULLs because it considers each NULL value distinct from other NULL values

Query 18: Retrieve the names of all employees who do not have supervisorsQ18: SELECT FNAME, LNAME

FROM EMPLOYEE

WHERE SUPERSSN IS NULL;

» Result of Q18

IM ISU 67Database

More Complex Queries (cont.) Aggregate Functions

Include COUNT, SUM, MAX, MIN, and AVG

Query 19: Find the sum of salaries, the maximum salary, the minimum salary, and the average salary among all employees

Q19: SELECT SUM(SALARY), MAX(SALARY), MIN(SALARY),

AVG(SALARY) FROM EMPLOYEE;

IM ISU 68Database

More Complex Queries (cont.) Queries 21 and 22: Retrieve the total number of employees in the company (Q21), and the number of employees in the 'Research' department (Q22)Q21: SELECT COUNT (*)

FROM EMPLOYEE;

Q22: SELECT COUNT (*)

FROM EMPLOYEE, DEPARTMENT

WHERE DNO=DNUMBER AND DNAME='Research’;

IM ISU 69Database

More Complex Queries (cont.) Grouping

In many cases, we want to apply the aggregate functions to subgroups of tuples in a relation

Each subgroup consists of the tuples having the same value for the grouping attribute(s)

The function is applied to each subgroup independently

SQL has a GROUP BY-clause for specifying the grouping attributes, which must also appear in the SELECT-clause

IM ISU 70Database

More Complex Queries (cont.) Query 24: For each department, retrieve the department number, the number of employees in the department, and their average salaryQ24: SELECT DNO, COUNT

(*), AVG (SALARY)

FROM EMPLOYEEGROUP BY DNO;

» In Q24, the EMPLOYEE are divided into groups having the same value for attribute DNO

» The COUNT and AVG functions are applied to each subgroup of tuples separately

IM ISU 71Database

More Complex Queries (cont.) Illustration of Q24

IM ISU 72Database

More Complex Queries (cont.) Query 25: For each project, retrieve the project

number, project name, and the number of employees working on that projectQ25: SELECT PNUMBER, PNAME,

COUNT (*)

FROM PROJECT, WORKS_ON

WHERE PNUMBER=PNO

GROUP BY PNUMBER, PNAME;» In this case, the grouping and functions are applied

after the joining of the two relations

IM ISU 73Database

More Complex Queries (cont.) The HAVING-CLAUSE

Sometimes we want to retrieve the values of functions for only those groups that satisfy certain conditions

The HAVING-clause is used for specifying a selection condition on groups (rather than onindividual tuples)

IM ISU 74Database

More Complex Queries (cont.) Query 26: For each project on which more than

two employees work, retrieve the project number, project name, and the number of employees Q26: SELECT PNUMBER, PNAME,

COUNT (*)

FROM PROJECT, WORKS_ON

WHERE PNUMBER=PNO

GROUP BY PNUMBER, PNAME

HAVING COUNT (*) > 2;

IM ISU 75Database

More Complex Queries (cont.) Illustration of Query 26

IM ISU 76Database

More Complex Queries (cont.) Illustration of Query 26

IM ISU 77Database

More Complex Queries (cont.) Summary of SQL Queries

A query in SQL can consist of up to six clauses, but only the first two, SELECT and FROM, are mandatory, ordered as followsSELECT <attribute list>

FROM <table list>

[WHERE <condition>]

[GROUP BY <grouping attribute(s)>]

[HAVING <group condition>]

[ORDER BY <attribute list>]

IM ISU 78Database

More Complex Queries (cont.) A query is evaluated by » first applying the FROM-clause,

» followed by the WHERE-clause,

» then GROUP BY and HAVING, and

» finally the SELECT-clause

IM ISU 79Database

Specifying Updates in SQL SQL commands to modify the database

INSERT, DELETE, and UPDATE

INSERT Command In its simplest form, it is used to add one or m

ore tuples to a relation Attribute values should be listed in the same o

rder as the attributes were specified in the CREATE TABLE command

IM ISU 80Database

Specifying Updates in SQL (cont.) Example: Add a new tuple to the EMPLOYEE

relationU1: INSERT INTO EMPLOYEE

VALUES ('Richard','K','Marini',

'653298653', '30-DEC-52',

'98 Oak Forest,Katy,TX',

'M', 37000,'987654321', 4);

IM ISU 81Database

Specifying Updates in SQL (cont.) An alternate form of INSERT specifies explicitly t

he attribute names that correspond to the values in the new tuple

Attributes with NULL values can be left out Example: Insert a tuple for a new EMPLOYEE for

whom we only know the FNAME, LNAME, and SSN attributesU1A: INSERT INTO EMPLOYEE (FNAME,

LNAME, SSN)

VALUES ('Richard', 'Marini', '653298653')

IM ISU 82Database

Specifying Updates in SQL (cont.) Note: Only the constraints specified in the DDL

commands are automatically enforced by the DBMS when updates are applied to the database

If a system does not support some constraint, the users must enforce the constraintU2: INSERT INTO EMPLOYEE (FNAME,

LNAME, SSN, DNO)

VALUES (‘Robert’, ‘Hatcher’, ‘980760540’, 2);

(* U2 is rejected if referential integrity checking is provided by DBMS *)

IM ISU 83Database

Specifying Updates in SQL (cont.) A DBMS enforcing NOT NULL will reject an

INSERT command in which an attribute declared to be NOT NULL does not have a value

U2A: INSERT INTO EMPLOYEE (FNAME, LNAME, DNO)

VALUES (‘Robert’, ‘Hatcher’, 5);

(* U2A is rejected if NOT NULL checking is provided by DBMS *)

IM ISU 84Database

Specifying Updates in SQL (cont.) Another variation of INSERT allows insertion o

f multiple tuples resulting from a query into a relation

Example: Suppose we want to create a temporary table that has the name, number of employees, and total salaries for each department. A table DEPTS_INFO is created by U3A, and is loaded with the summary information retrieved from the database by the query in U3B

IM ISU 85Database

Specifying Updates in SQL (cont.) U3A: CREATE TABLE DEPTS_INFO

(DEPT_NAME VARCHAR(10),

NO_OF_EMPS INTEGER,

TOTAL_SAL INTEGER);

U3B: INSERT INTO DEPTS_INFO (DEPT_NAME,

NO_OF_EMPS, TOTAL_SAL)

SELECT DNAME, COUNT (*), SUM (SALARY)

FROM DEPARTMENT, EMPLOYEE

WHERE DNUMBER=DNO

GROUP BY DNAME ;

IM ISU 86Database

Specifying Updates in SQL (cont.) DELETE Command

Removes tuples from a relation Includes a WHERE-clause to select the tuples to

be deleted Tuples are deleted from only one table at a time

(unless CASCADE is specified on a referential integrity constraint)

A missing WHERE-clause specifies that all tuples in the relation are to be deleted

IM ISU 87Database

Specifying Updates in SQL (cont.) Examples:

U4A: DELETE FROM EMPLOYEE

WHERE LNAME='Brown’

U4B: DELETE FROM EMPLOYEE

WHERE SSN='123456789’

U4C: DELETE FROM EMPLOYEE

WHERE DNO IN (SELECT DNUMBER

FROM DEPARTMENT

WHEREDNAME='Research')

U4D: DELETE FROM EMPLOYEE

IM ISU 88Database

Specifying Updates in SQL (cont.) UPDATE Command

Used to modify attribute values of one or more selected tuples

A WHERE-clause selects the tuples to be modified

An additional SET-clause specifies the attributes to be modified and their new values

Each command modifies tuples in the same relation

Referential integrity should be enforced

IM ISU 89Database

Specifying Updates in SQL (cont.) Example: Change the location and controlling

department number of project number 10 to 'Bellaire' and 5, respectively.

U5: UPDATE PROJECT

SET PLOCATION = 'Bellaire', DNUM = 5

WHERE PNUMBER=10

Example: Give all employees in the 'Research' department a 10% raise in salary.

IM ISU 90Database

Specifying Updates in SQL (cont.) U6: UPDATE EMPLOYEE

SET SALARY = SALARY *1.1

WHERE DNO IN (SELECT DNUMBER

FROM DEPARTMENT

WHERE DNAME='Research')

» In this request, the modified SALARY value depends on the original SALARY value in each tuple

IM ISU 91Database

Relational Views in SQL Basic concepts

A view is a single virtual table that is derived from other tables, which could be base tables or previously defined views

A view does not necessarily exist in physical form; that means the view does not store any tuples

There are limitations on possible update operations that can be applied to views

There is no limitation on querying a view

IM ISU 92Database

Relational Views in SQL (cont.) Specify Views

Using CREATE VIEW command The view attribute names can be inherited from the

attribute names of the tables in the defining query Examples:

V1:CREATE VIEW WORKS_ON1 AS SELECT FNAME, LNAME, PNAME, HOURS FROM EMPLOYEE, PROJECT, WORKS_ON

WHERE SSN=ESSN AND PNO=PNUMBER;

IM ISU 93Database

Relational Views in SQL (cont.) V2:CREATE VIEW DEPT_INFO

(DEPT_NAME, NO_OF_EMPS, TOTAL_SAL) AS

SELECT DNAME, COUNT (*), SUM (SALARY)

FROM DEPARTMENT, EMPLOYEE

WHERE DNUMBER=DNO

GROUP BY DNAME;

IM ISU 94Database

Relational Views in SQL (cont.) Queries on Views

A view can be defined to simplify frequently occurring queries

Example: Retrieve the last name and first name of all employees who work on 'ProjectX'.QV1: SELECT PNAME, FNAME, LNAME

FROM WORKS_ON1

WHERE PNAME='ProjectX';

» Without the view WORKS_ON1, this query specification would require two join conditions

IM ISU 95Database

Relational Views in SQL (cont.) A view is always up-to-date

» If the base tables on which the view is defined are modified, the DBMS is responsible for keeping it up to date

» Hence, the view is not realized at the time of view definition, but rather at the time we specify a query on the view

A view is removed using the DROP VIEW command» Example:

V1A: DROP VIEW WORKS_ON1;

IM ISU 96Database

Relational Views in SQL (cont.) Updating of Views

A view update operation may be mapped in multiple ways to update operations on the defining base relations

The topic of updating views is still an active research area

Example: Suppose we issue the command in UV1 to update the WORKS_ON1 view by modifying the PNAME attribute of 'John Smith' from 'ProductX' to 'ProductY'.

IM ISU 97Database

Relational Views in SQL (cont.) UV1: UPDATE WORKS_ON1

SET PNAME = 'ProductY' WHERE LNAME='Smith' AND FNAME='John'

AND PNAME='ProductX’» This can be mapped into several updates on the base

relations to give the desired update on the view» Two possibilities

1. Change the name of the 'ProductX' tuple in the PROJECT relation to 'ProductY’

2. Relate 'John Smith' to the 'ProductY' PROJECT tuple in place of the 'ProductX' PROJECT tuple

IM ISU 98Database

Relational Views in SQL (cont.) » Method 1

UPDATE PROJECTSET PNAME = 'ProductY'

WHERE PNAME = 'ProductX’

– This has the effect of changing all the view tuples with PNAME = ‘ProductX’

– It is quite unlikely that the user who specified view update UV1 wants the update to be interpreted this way

IM ISU 99Database

Relational Views in SQL (cont.) » Method 2

UPDATE WORKS_ONSET PNO = (SELECT PNUMBER

FROM PROJECT WHERE PNAME='ProductY')

WHERE ESSN IN (SELECT SSN FROM EMPLOYEE

WHERE LNAME='Smith' AND FNAME='John') AND

PNO IN (SELECT PNUMBER FROM PROJECT

WHERE PNAME='ProductX')

IM ISU 100Database

Relational Views in SQL (cont.) Some view updates may not make much sense

» Example: Modify the TOTAL_SAL attribute of DEPT_INFO as in UV2

UV2: UPDATE DEPT_INFO

SETTOTAL_SAL=100000

WHERE DNAME='Research'; In general, we cannot guarantee that any view can

be updated A view update is feasible only if one update on the

base relations can accomplish the desired update effect on the view

IM ISU 101Database

Relational Views in SQL (cont.) If a view update can be mapped to more than

one update on the underlying base relations, we have to choose the desired update

General guideline» A view with a single defining table is updatable if

the view attributes contain the primary key» Views defined on multiple tables using joins are

generally not updatable» Views defined aggregate functions are not updatable

DatabaseIM ISU1 Fundamentals of Database Systems Chapter 8 SQL99 - The Relational Database Standard.

Documents

Transcript of DatabaseIM ISU1 Fundamentals of Database Systems Chapter 8 SQL99 - The Relational Database Standard.