08-Unit8

Database Management Systems Unit 8

Sikkim Manipal University Page No.: 128

Unit 8 History of SQL

Structure

8.1 Introduction

Objectives

Self Assessment Question(s) (SAQs)

8.2 Data Retrieval Statement (SELECT)


8.3 Multi table Queries

8.3.1 Nested Queries or Sub queries

8.3.2 Multiple Row Nested Queries:

8.3.3 The Exists Clause


8.4 Data Manipulation Language


8.5 The Create Table Statement


8.6 Summary

8.7 Terminal Questions (TQs)

8.8 Multiple Choice Questions (MCQs)

8.9 Answers to SAQs, TQs, and MCQs

8.9.1 Answers to Self Assessment Questions (SAQs)

8.9.2 Answers to Terminal Questions (TQs)

8.9.3 Answers to Multiple Choice Questions (MCQs)

8.1 Introduction

The history of SQL began in an IBM laboratory in San Jose, California,

where SQL was developed in the late 1970's. SQL stands for structured

Query Language. It is a non-procedural language, meaning that SQL

describes what data to retrieve, delete or insert, rather than how to perform



the operation. It is the standard command set used to communicate with

the RDBMS.

A SQL query is not-necessarily a question to the database. It can be

command to do one of the following.

Create or delete a table.

Insert, modify or delete rows.

Search several rows for specifying information and return the result in

order.

Modify security information.

THE SQL STATEMENT CAN BE GROUPED INTO FOLLOWING

CATEGORIES.

1. DDL(Data Definition Language)

2. DML(Data Manipulation Language)

3. DCL(Data Control Language)

4. TCL(Transaction Control Language)

DDL: Data Definition Language

The DDL statement provides commands for defining relation schema i,e for

creating tables, indexes, sequences etc. and commands for dropping,

altering, renaming objects.

DML: (Data Manipulation Language)

The DML statements are used to alter the database tables in someway.

The UPDATE, INSERT and DELETE statements alter existing rows in a

database tables, insert new records into a database table, or remove one or

more records from the database table.

DCL: (Data Control Language)



The Data Control Language Statements are used to Grant permission to the

user and Revoke permission from the user, Lock certain Permission for the

user.

SQL DBA>Revoke Import from Akash;

SQL DBA>Grant all on emp to public;

SQL DBA>Grant select, Update on EMP to L.Suresh;

SQlDBA>Grant ALL on EMP to Akashwith Grant option;

Revoke: Revoke takes out privilege from one or more tables or views.

SQL DBA>rEOKE UPDATE, DELETE FROM l.sURES;

SQL DBA>Revoke all on emp from Akash

TCL: (Transaction Control Language)

It is used to control transactions.

Eg: Commit

Rollbakc: Discard/Cancel the changes upto the previous commit point.

SQL* COMMANDS:

This subsection discusses the often used commands in sql environment.

For example, if your SQL commands are saved in a file (typically in note

pad) you can execute this file using an "at" @command, similarly there are a

number of such commands:

@<filename> Runs the command file stored in <filename>

/ Runs the SQL command or PL/SQL block

currently stored in the SQL buffer

EXEC[UTE] Runs a single PL/SQL statement.

R[UN] Runs the SQL command or PL/SQL block

currently stored in the SQL buffer.

R<filename> Runs the file specified in <filename>

EXIT or QUIT Exits from SQL.

LIST Lists the content of the buffer



A[PPEND] <text> Adds text at the end of the line

CLEAR BUFFER Deletes all the lines in the buffer

GER<filename> Loads host OS file into SQL BUFFER

(does not execute it)

SAV[E]<filename> Saves contents of buffers to OS file.

DEFIN_EDITOR='notepad' Define notepad as the editor.

ED[IT] Invokes the editor defined through

DEFINE_EDITOR>

DATA TYPES IN ORACLE 8i SQL:

The fig. shows the complete listing of the data types allowed in oracle.

DATA TYPE DESCRIPTION

CHAR (sizs) Fixed length character. Max = 2000

VARCHAR2(size) Variable length character. Max=4000

DATE Date, valid range is from jan1,4712 B.C to.DEC 31,4712 A.D.

BLOB Binary large object Max =4GB

CLOB Character large object Max=4G.B.

BFILE Pointer to binary OS file

LONG Character data of variable size, Max=2G.B.

LONG RAW Raw binary data. Rest is same as long

NUMBER (size) Numbers. Max. size =40 digits

NUMBER(size,d) Numbers, range=1.0E-130 tto 9.9E125

DECIMAL Same as NUMBER. Size /d can't be specified

FLOAT Same as NUMBER

INTEGER Same as NUMBER Size /d can't be specified

SMALLINT Same as NUMBER

EXAMPLE TABLES:

To study the SQL commands of various types we need some tables. Let us

consider two tables shown in the fig., which will be used throughout our

discussion.



EMPLOYEE

SSN NAME BDATE SALARY MgrSSN Dno

1111 Deepak 5-jan-62 22000 4444 1

2222 Yadav 27-feb-84 30000 4444 3

3333 Venkat 22-jan-65 18000 2222 2

4444 Prasad 2-feb-68 32000 Null 3

5555 Reena 4-aug-79 8000 4444 3

DEPARTMENT

DNO DNAME LOC

1 Admin Chennai

2 Research Bangalore

3 Accounts Bangalore

Objectives

To Know About

o Data Retrieval Statement (SELECT)

o Multi table Queries

o Nested Queries or Sub queries:

o Multiple Row Nested Queries:

o The Exists Clause

o Data Manipulation Language

Self Assessment Question(s) (SAQs) (For Section 8.1)

1. What is SQL?

2. Write down the categories into which a SQL statement can be grouped?

3. Explain the concept of DCL.

8.2 Data Retrieval Statement (SELECT)

The select statement is used to extract information from one or more tables

in the database. To demonstrate SELECT we must have some tables



created within a specific user session. Let us assume Scott as the default

user and create two tables Employee and Department using CREATE

STATEMENT

CREATE TABLE Department(

Dno number(d) not null

Dname varchar2(10) not null

Loc varchar2(15)

Primary key (Dno));

Create table employee (

SSN number (4) not null

Name varchar3=2(20) not null

Bdate date,

Salary number(10,2)

MgrSSN umber(4)

DNo number(2) not null

Primary key(SSN)

Foreign key [MgrSSN] reference employee(SSN)

Foreign key (DNo) reference department (DNo))

The syntax of SELECT STATEMENT is given below:

Syntax:

Select* | {[DISTINCT] column | expression \)

From table(s);

The basic select statement must include the following;

A SELECT clause

A FROM clause

Selecting all columns

Example – 1

Select * From employee



The * indicates that it should retrieve all the columns from employee

table. The out put of this query shown below:

Out put-1

Dno SSN NAME BDATE SALARY MgrSSN

1 1111 Deepak 5-jan-62 20000 4444

3 2222 Yadav 27-feb-60 30000 4444

2 3333 Venkat 22-jan-65 18000 2222

3 4444 Prasad 2-feb-84 32000 Null

3 5555 Reena 4-aug-65 8000 4444

Example – 2

SELECT * FROM employee

ORDER BY SSN;

Output-2

SSN NAME BDATE SALARY MgrSSN Dno

1111 Deepak 27-feb-84 30000 4444 3

2222 Yadav 15-jan-65 8000 4444 1

3333 Venkat 22-jan-85 20000 2222 2

4444 Prasad 27-feb-84 32000 Null 3

5555 Reena 15-jan-65 8000 4444 1

SELECTING SPECIFIC COLUMNS:

We wish to retrieve only name and salary of the employees.

Example-3

SELECT name, salary FROM employee:

OUT PUT-3

NAME SALARY

Prasad 32000

Reena 8000

Deepak 22000

Venkat 30000

Yadav 18000



Using arithmetic operators:

SELECT name, salary, salary * 12

FROM employee;

Out put-4

NAME SALARY SALARY *12

Prasad 32000 384000

Reena 8000 96000

Deepak 22000 264000

Yadav 18000 360000

Venkat 30000 216000

USING ALIASES(Alternate name given to columns):

SELECT \(Name, Salary, Salary *12 "YRLY SALARY"

FROM Employee

OUT PUT

NAME SALARY SALARY *12

Prasad 32000 384000

Reena 8000 96000

Deepak 22000 264000

Yadav 18000 360000

Venkat 30000 216000

Eliminating duplicate rows:

To eliminate duplicate rows simply use the keyword DISTINCT.

SELECT DISTINCT MGRSSN FROM Employee

OUTPUT

MGRSSN

2222

4444

DISPLAYING TABLE STRUCTURE:

To display schema of table use the command DESCRIEBE or DESC.

DESC Employee;



OUTPUT

NAME NULL? TYPE

Ssn NOT NULL NUMBER [4]

NAME NOT NULL VARCHAR2 [20]

BDATE DATE

SALARY NUMBER [10,20]

MGRSSN NUMBER [4]

DNO NOT NULL NUMBER [2]

SELECT statement with WHERE clause

The conditions are specified in the where clause. It instructs sql to search

the data in a table and returns only those rows that meet search criteria.

SELECT * FROM emp

WHERE name = 'yadav'

Out put-1

SSN NAME BDATE SALARY MGRSSN DNO

2222 yadav 10-dec-60 30000 4444 3

SELECT Name, Salary FROM Employee

WHERE Salary > 20000;

OUT PUT

NAME SALARY

Prasad 32000

Deepak 22000

Yadav 18000

RELATIONAL OPERATOR S AND COMPARISON CONDITIONS:

= Equal to

> Greater than

>= Greater than or equal

< Less than



<= Less than or equal

<> Not equal

BETWEEN<a>ABD<b> Range between <a> and <b> inclusive

1N<set> True when the member is in the <set>

LIKE<pattern> Matches a specified pattern

IS NULL Is a null value

BETWEEN AND OR OPERATOR:

To illustrate the between and or operators.

SQL supports range searches. For eg If we want to see all the employees

with salary between 22000 and 32000

SELECT * from employee

WHERE salary between 22000 and 32000

Example

NAME SALARY

Prasad 32000

Yadav 30000

Deepak 22000

IS NULL OR IS NOT NULL:

The null tests for the null values in the table

Example:

SELECT Name FROM Employee

WHERE Mgrssn IS NULL;

OUT PUT:

NAME

Prasad

SORTING (ORDER BY CLAUSE):

It gives a result in a particular order. Select all the employee lists sorted by

name, salary in descending order.



Select* from emp order by basic;

Select job, ename from emp order by joindate desc;

Desc at the end of the order by clause orders the list in descending order

instead of the default[ascending] order.

Example:

SELECT* FROM EMPLOYEE

ORDER BY name DESC.

OUT PUT:

NAME

Reena

Pooja

Deepak

Aruna

LIKE CONDITION:

Where clause with a pattern searches for sub string comparisons.

Sql also supports pattern searching through the like operator. We

describe patterns using two special characters.

Percent [%]. – The % character matches any sub string.

Underscore ( _ ) – The underscore matches any character.

Example:

SELECT emp_name from emp

WHERE emp_name like 'Ra%'

Example: 1

NAME

Raj

Rama

Ramana



Select emp_name from emp where name starts with 'R' and has j has

third caracter.

SELECT *from Emp

WHERE EMP_NAME LIKE 'r_J%';

WHERE clause using IN operator

SQL supports the concept of searching for items within a list; it compares

the values within a parentheses.

1. Select employee, who are working for department 10&20.

SELECT * from emp

WHERE deptno in (10,20)

2. Selects the employees who are working in a same project as that raja

works onl

SELECT eno from ep

WHERE (pno) IN (select pno from works_on where empno=E20);

AGGREGATE FUNCTIONS AND GROUPING:

Group by clause is used to group the rows based on certain common

criteria; for e.g. we can group the rows in an employee table by the

department. For example, the employees working for department number 1

may form a group, and all employees working for department number 2 form

another group. Group by clause is usually used in conjunction with

aggregate functions like SUM, MAX, Min etc.. Group by clause runs the

aggregate function described in the SELECT statement. It gives summary

information.

Example: For each department, retrieve the department number, the

number of employees in the department and their average salary.

SELECT Dno, count(*) "No.of Employees"

FROM employee

GROUP BY DNO;



OUT PUT

DNO No. of Employees

30 2

40 5

52 3

53 4

Select total salary for each department.

SELECT deptno sum(salary) from emp

GROU BY depno;

OUT PUT:

DNO SUM(SALARY)

1 22000

2 18000

3 7000

For each project, retrieve the project number, project name and the

number of employees who work on total project.

o SELECT Pnumber, Pname, count(*)

FROM project, works on

WHERE Pnumber=PNO

GROUP BY P number, Pname;

Retrieve total number of employees in the research department.

o SELECT COUNT(*) FROM employee, department

WHERE Dno=Dnumber and

Dname='Research'

Find the sum of salaries, the maximum and minimum salary of all the

employees.

o SELECT Sum(salary), max(salary),

Min(salary) FROM emp;



Sum(salary) Mad(salary) Min(salary)

50000 32000 8000

Find the sum of the salaries of all employees of the 'Research'

department, as well as the maximum salary, minimum salary in this

department.

o SELECT SUM(salary), max(salary), min(salary)

FROM emp, department

Where DNO=Dnumber and

Dname='Researhc';

HAVING CLAUSE:

The having clause filtrs the rows returned by the group by clause.

Eg: 1>Select job, count(*)from emp group by job having count(*)>20;

2>Select Deptno,max(basic), min(basic)from emp groupby

Detno having salary>30000

find the average salary of only department 1.

SELECT DnO,avg(salary)

FROM Employee

GROUP BY Dno

HAVING Dno = 1;

For each department, retrieve the department number, Dname and

number of employees working in that department, so that department

should contain more than three employees.

o SELECT Dno, Dname, count(*)

FROM Emp, Dept.

WHERE Emp.Dno=dept.Dno

GROUP BY Dno

HAVING count(*)3;

Here where_clause limits the tuples to which functions are applied, the

having clause is used to select individual groups of tuples.



For each department that has more than three employees, retrieve the

department number and the number of its employees, who are earning

more than 10,000.

o Example::

SELECT Dno, AVG(salary)

FROM Employee

WHERE Bdate LIKE '%jan%'

GROUP BYDno

HAVING max(salary) > 10000;

OUT PUT:

DNO AVG(SALARY)

1 22000

2 18000

3 20000

Self Assessment Question(s) (SAQs) (For section 8.2)

1. How do you retrieve data from the database?

2. Explain the different types of relational operators used in retrieval of

data.

3. Explain the concept of Sorting.

4. How do you perform searching in SQL?

5. Write a note on aggregate functions and grouping in SQL.

8.3 Multi table Queries

So far the queries that we have discussed were containing only one table in

the from clause. There are many occasions in the database applications

where we need to retrieve data from more than one table. This section

addresses these kind of queries.

SIMPLE EQUI-JOINS:

When two tables are joined together we must follow these guidelines:

Table names in the FROM clause are separated by commas.



Use appropriate joining condition. This means that the foreign key of

table 1 will be made equal to the primary key of table 2. This column

acts as the joining attribute. For example, dno of employee table and

dno of department will be involved in the joining condition of WHERE

clause.

EXAMPLE-1: This example demonstrates the equijoin and the purpose

is to display the employee names and the department names for which

they work.

SELECT NAME, DNAME

FROM Employee, Department

WHERE employe.Dno = department.Dno;

OUTPUT:

NAME DNAME

Prasad Accounts

Reena Accounts

Deepak Admin

Venkat Accounts

Pooja Research

EXAMPLE 2:

Let us now try to display only employees working for Accounts

department.

SELECT Name, salary, Dname

FROM Employee, department

WHERE (Emplyee.DNO = Department.DNO)

AND (Dname = 'Accounts');

OUT PUT:

NAME SALARY DNAME

Prasad 32000 Accounts

Reena 8000 Accounts

Venkat 30000 Accounts



SELF JOIN and TABLE ALIASES:

The self-join is one where you involve the same table in the join. This is

illustrated in the following example. This technique is used fully to solve

many queries.

To find the employee who earns more than venkat

SELECT e1.name, e1.salary

FROM Employee e1, Employee e2

WHERE (e1.salary > e2.salary) AND (e2.name = 'venkat')

OUT PUT:

NAME SALARY

Prasad 32000

OUTER JOINS:

Outer joins are used to display rows that do not meet the join condition. For

left outer join use a plus sign(+) to left condition and for right outer join use

the plus sign to the right condition. The syntax for left and right outer joins

are given below:

Left outer join

SELECT table1.col, table2.col

FROM table1 t1, table2 t2

WHERE t1.col(+) = t2.col;

Notice that the plus sign cannot be placed on both sides of the condition.

EXAMPLE 1: This example demonstrates the right outer join by retaining

the right side table(department) tuples and giving null values for the

tuples that do not match the left side table (employee).

SELECT Name, Dname

FROM Employee E, Department D

WHERE E.Name(+) =D.Dname;



OUTPUT::

NAME DNAME

Accounts

Admin

EXAMPLE 2: This is same as ex.1, but the only difference is that it is a

left outer join. So all the left table (employee) rows are kept, and if no

match occurs with the right side table (department) a null is shown.

SELECT Name, Dnaem

FROM Employee E, Department D

WHERE E.Name = D.Dname(+);

OUT PUT:

NAME DNAME

Deepak

Venkat

Pooja

Prasad

Reena

8.3.1 Nested Queries or Sub queries

A where clause generally contains a condition; but it can also contain an sql

query. The query within a WHERE clause is called the inner query or sub

query, and the query that encloses the inner one is called as outer query or

main query. It is also possible to place the inner query with in a FROM or

HAVING clause. Using nested queries it is possible to build powerful sql

programs.

Execution of nested queries:

The general syntax of a nested query is given below.

SELECT <column (s)>

FROM table outer query



WHERE <condn> operator

(SELECT <column>

FROM table); inner query

The operator mentioned in the outer query can be any one of >, =, or IN.

Normally the outer query uses the result of the inner query to display the

values of columns mentioned in the outer query.

Single-Row Nested Queries

The simplest single-row nested query is by using = sign.

EXAMPLE: Assume that we wish to display the names and the

employees working for accounts department.

SELECT Name

FROM Employee

WHERE Dno =

(SELECT DNo

FROM Department

WHERE Dname = 'accounts');

OUTPUT:

NAME

Prasad

Reena

Venkat

GROUP BY clause in SUB QUERIES:

Display all the employees drawing more than or equal to the average salary

of department number 3.

SELECT Name, Salary

FROM Employee

WHERE Slary >=



(SELECT AVG(salary)

FROMEmployee

GROUP BY Dno

HAVING dnO = 3);

OUT PUT

NAME SALARY

Prasad 32000

Venkat 30000

8.3.2 Multiple Row Nested Queries

The operators IN, ANY, and ALL are used in the multiple row sub queries.

The sub query in this case returns more than one row.

OPERATORS DESCRIPTION

IN Equal to any member in the list

ANY Compare value to each value returned by the sub query

ALL Compare value to all the values returned by the sub query.

EXAMPLE: Display the name of the highest paid employee,

SELECT Name, salary

FROM Employee

WHERE Salary IN

(SELECT MAX(Salary)

FROMEmployee)

Remember that the multiple row sub queries expect one or more results. In

this example the inner query gives a single value and the next example

shows a set of values. The following table gives an idea of how to use ANY

and ALL.



OPERATOR MEANING EXAMPLE

<ANY Less than the maximum

e<ANY(5.3.8) e is less than any single item in the life (5,3,8). Even 7 qualifies, because 7<8

>ANY More than the minimum

e>ANY (5,3,8): e is less than any single item in the list (5,3,8). Even 4 qualities, because 4>3.

<ANY Same as IN e = any (5,3,8). ALL value in the list quality,

<ALL Less than the maximum

e <ALL (5,3,8); anything below 3 qualifies.

>ALL More than the maximum

e > ALL (5,3,8) : anything greater than 8 qualifies

!=ALL Not equal to any thing

E !=(5,3,8): any thing other than 5, 3 and 8 qualifies.

EXAMPLE 1:

SELECT Name, salary

FROM Employee

WHERE Salary< ANY

(SELECT Salary

FROMEmployee

WHERE DNo =3);

OUTPUT:

NAME SALARY

Reena 8000

Deepak 22000

Venkat 30000

Pooja 18000

EXAMPLE 2:

SELECT Name, salary

FROM Employee

WHERE Salary> ANY

(SELECT Salary

FROMEmployee

WHERE DNO =3);



OUTPUT:

NAME SALARY

Prasad 32000

Deepak 22000

Venkat 30000

Pooja 18000

In this example, all the rows qualify except the row with salary 8000,

because all employees draw more than the minimum salary in the result

of the sub query. If your condition is >= then you get all the rows.

EXAMPLE

SELECT Name, salary

FROM Employee

WHERE Salary< ANY

(SELECT Salary

FROMEmployee

WHERE DNO =3);

If any body draws a salary lower than the minimum value in the set, their

names will be displayed. Here, nobody draws lower than 8000 and

hence there is no output.

OUTPUT:

No rows selected.

EXAMPLE2:

SELECT Name, salary

FROM Employee

WHERE Salary > ALL

(SELECT Salary

FROMEmployee

WHERE DNO =3);



Similar to the previous examples - nobody draws more than the

maximum in the set and so no output again.

OUTPUT:

No rows selected.

EXAMPLE:

SELECT Name, salary

FROM Employee

WHERE Salary = ALL

(SELECT Salary

FROMEmployee

WHERE DNO =3);

Obviously this example should output salaries of employees other than

the set given by the subquery.

OUTPUT:

NAME SALARY

Deepak 22000

Pooja 18000

8.3.3 The Exists Clause

The exits clause returns true in a WHERE clause, if the subquery that

follows returns at least one row.

EXAMPLE:

Assume that we want to display the names of employees who work for the

Accounts department; it can be written as:

SELECT Name

FROM Employee E

WHERE EXISTS

(SELECT*FROM Department D

SHERE E.DNO = D.DNO AND DNAME = 'Accounts')



OUTPUT:

NAME

Prasad

Reena

Venkat


1. Write down the guidelines used to query more than one table.

2. What do you mean by nested query?

3. Define sub query and give one example

4. Explain the concept of multiple row nested queries.

5. What does Exists clause return?

8.4 Data Manipulation Language

A data manipulating language [DML] consists of SQL statements that are

used to insert, delete and update the records in a table.

INSERT STATEMENT:

To add a new row into the table you can follow the syntax:

INSERT INTO table [(column-1I, column-2…I)]

Values (value- I I, value-2…..I);

Using this syntax you can insert only one row at a time. To insert more than

one row, you can execute the insert statement repeatedly. The simplest

example for INSERT statement is shown below.

EMPLOYEE

INSERT INTO Employee

VALUES (1111, 'deepak', '5-jan-82', 0000, 4444,);

To enter more records we can use / (slash symbol)

'/' is used to execute the commands stored in the buffer

insert into emp(empno,eaddr,basic)

values(&empno,'&eaddr',&ba);



DELETE OMMAND

It is a DML statement to delete record(s)

Syntax: DELETE FROM table

[WHERE cond]; / / If the WHERE condition is not present in

the query, all the rows in the table are deleted.

Example: delete from emp where name = 'Yadav';

UPDATE COMMAND

It is used to change existing values in a table.

Syntax: UPDATE table

SET col I = val I, col2 = val2……

[WHERE cond];

update emp set deptno = 100; / / If the WHERE condition is not present in

the query, all the rows in the table are upaed.

Update emp set ename= 'Sourav' where empno = 100;

Transaction Control Language It is used to control transaction

Eg: Commit

Save changes permanently in the database.

Roll back: Discard/Cncel the changes upto the previous commit point.

Save point:

Is used to commit / Rollback particular point.

Ex: Commit………………

Insert…………………

Update………………..

Savepoint aa

Delete

#

#

#

Rollback to aa.

Commit to a



Creating And Altering Database objects (DDL):

The basic notion of this section is to introduce the ways to create and

manipulate the following database objects.

Table: A tabular structure that stores data.

View: A tabular structure similar to a table but it is a

collection of one or more tables.

Sequence Automatically generates a sequence of numbers

Index: Provides an efficient access structure.


1. How do you perform insertion, deletion and update operations in SQL?

8.5 The Create Table Statement

Now we shall discuss in detail ways to create and alter tables with

constraints.

syntax: CREATE TABLE tablename

(column_1 datatype,column_2 datatype………….);

You specify the name of the table (should be unique) and one or

more attributes and their data types.

EXAMPLE

CREATE TABLE Employee (

SSN number (4) not null

NAME varchar (2) (20) not null

BDATE data,

Mgrssn number,

Primary key(SSN)

Foreign key(mgrssn ) reference emplyee(SSN));

Alter Table Statement



After creating a table, one or more columns can be added to this table,

similarly, columns can be dropped [oracle 9i only) and in either case the

existing table columns will not be affected, for example, assume that we

wish to add a column phone to employee table

EXAMPLE

ALTER TABLE Employee

ADD phone number(7) not null;

Using the same alter command you can modify the data type of a

column.

For example, the phone column can be modified from number to

varchar2.

ALTER TABLE Employee

MODIFY phone varchar2(10);

Oracle 81 does not support dropping a column, but oracle 9i does it.

ALTER TABLE employee

DROP COLUMN MODIFY phone;

Dropping a table even when it has data is possible

SYNTAX: DROP [TABLE] table;

For Example, to drop the employee table, use the following statement.

DROP employee

To rename the table use the RENAME statement as shown below

RENAME Employee to workers.

VIEWS (Virtual Table)

View is a derived table, which doesn't have storage of its own. Views are

created by picking certain columns from the base table. The advantage of

using views are:

It restricts direct data access form tables I,e it provides security

Reduces joining of tables each and every time.



Syntax: Create viw<view name> as select <column/s> from<table/s>

where<condition>

Example: Create view V1 as Se;ect ssm.ma,e.sa;aru frp, e,[;

Desc V1;

You can create views by referring to more than 1 table.

NOTE:

1. If there are NOT NULL columns which are missing in view, you cannot

insert the records.

2. If a view is created by referring to more than 1 table we cannot do DML

operation except select.

3. View updation (Ins.Del.Update) is possible only if it is created by a single

table.

Insert into V1 values (11,'LSURESH',50000)

Update v1 set ename='Akash' where empno=101;

Create view v2 as select empno, ename,dept.deptno.deptname from

emp.dept;

Indexing:

> Indexing provides a faster access (for to columns that are indexed)

> Indexes can also be used to ensure that no duplicate values are

entered into a column.

Eg: Primary key of a table

SYNTAX:

CREATE INDEX index_name

ON table (column1,column2……);

Example:

Create index ind 1 on emp(empno);

1. Query 1

Retrieve the name and address of all employee who work for the

research department.



1. SELECT FNAME,LNAME,ADDRESS

FROM EMPLOYEE,DEPARTMENT

WHERE DNAME = 'research' AND D number=DNO

Query Q1 is similar to a SELECT – PROJECT – JOIN sequence of

relational algebra operations.

Such queries are often called select-project-join queries. In the WHERE

clause of Q1, the conditional DNAME = 'Research' is the selection condition

and corresponds to a SELECT operation in Relational algebra.

Other Important Examples:

Company database example

This example uses the following tables and underlines columns that are the

primary keys

Employee(

SSN char (9), Name varchar2 (10), Bdate Date, Address varchar 2 (30),

Sex chart (1), Salary Number (10, 2) SuperSSN char(9), Dno Number (2))

Department(

Dnumber Number(2), Dname Varchar2(10), MgrSSj char(9), Mgrstartdate

Date)

Project(

Pnumber Number(2), Pname vrchar2(10). Plocation varchar2(15). Dnum

Number(2))

Depedent(

ESSN CHAR(9), Dependent name Varchar2(15), sex char, Bdate Date,

Relationship varchar2(10))

Dept_locations(

Dnumber Number(2), Dlocation varchar2(15))

Works_on(



ESSN char(9), Pno Number(2), Hours Number(3,1))

Query 1:

Retrieve the name and address of all employees who work for the research

department.

Q1: SELECT FNAME,LNAME,ADDRESS

FROM EMPLOYEE,DEPARTMENT

WHERE DNAME = 'research' AND Dnumber=DNO

QUERY 2:

Retrieve the birthdate and address of the employee whose name is 'John B.

Smith'.

Q2: SELECT BDATE,ADDRESS

FROM EMPLOYEE

WHERE FNAME = 'John ' AND D minit='B' and

LNAME='Smith'

This query involves only the 'EMPLOYEE' relation listed in the FROM

clause.

QUERY 3:

For every project located in 'stafford', list the project number, the controlling

department number, and the department manager's last name, address and

birth date.

Q3: SELECT PNUMBER,DNUM,LNAME,ADDRESS,BDATE

FROM PROJECT,DEPARTMENT,EMPLOYEE

WHERE DNUM=DNUMBER AND MGRSSN=SSN AND

PLOCATION='Stafford'

The join condition DNUM=DNUMBER relates a project to its controlling

department, where as, the join condition MGRSSN=SSN relates the

controlling department to the Employee, who manages that department.



QUERY 4:

Retrive the name of each employee who has a dependant with the same

first name as the employee.

Q4: SELECT E.FNAME,E.LNAME

FROM EMPLOYEE

WHERE E.SSN IN (SELECT ESSN FROM DEPENDENT

WHERE ESSN=E.SSN AND

E.FNAME=DEPENDENT_NAME)

QUERY 5: SELECT FNAME,LNAME

FROM EMPLOYEE

WHERE ( (SELECT PNO

FROM WORKS_ON

WHERE SSN=ESSN)

CONTAINS

(SELECT PNUMBER

FROM PROJECT

WHERE DNUM=5) )

QUERY 6:

List the names of managers who have at least one dependant.

SELECT FNAM.LNAME

FROM EMPLOYEE

WHERE EDISTS (SELECT *

FROM DEPENDENT

WHERE SSN=ESSN)

AND

EXISTS (SELECT *

FROM DEPENDENT

WHERE SSN=MGRSSN



One way to write this query is shown in Q7, where we specify two nested

correlates.

Queries: the first selects all dependent tuples related to an EMPLOYEE, and

the second selects all department tuples managed by the EMPLOYEE.

QUERY 7:

For each employee, retrieve the employee's first and last name of his or her

immediate supervisor.

Q7: SELECT E.NAME,E.LNAME,S.FNAME,S.LNAME

FROM EMPLOYEE E.EMPLOYEE S

WHERE E.SUPERSSN=S.SSN

In this case, we are allowed to declare alternative relation names E and S,

called aliases, for the EMPLOYEE relation.

QUERY 8:

Make a list of all project numbers for projects that involve an employee

whose last name is 'Smith', either as the worker or as the manager of the

department that controls the project.

Q8: SELECT PNUMBER

FROM PROJECT.DEPARTMENT.EMPLOYEE

WHERE DNUM=DNUMBER AND MGRSSN=SSN AND

LNAME='smith'.)

UNION

(SELECT PNUMBER

FROM WHERE PNUMBER=PNO AND ESSN=SSN

AND LNAME='smith'

The first SELECT query retrieves the project that involves a Smith as

manager of department that controls the project, and the second retrieves

the projects that involve a 'Smith' as a worker on the project.



QUERY 9:

Retrieve the social security numbers of all employees who work on project

numbers 1,2,3.

Q9: SELECT DISTINCT ESSN

FROM WORKS ON

WHERE PNO IN (1,2,3)

QUERY 10:

Retrieve the social security numbers of all employees who work on project

numbers 1,2,3.

Q.10: SELECT DISTINCT ESSN

FROM WORKS ON

WHERE PNO IN (1,2,3)

QUERY 11:

Find the sum of all salaries of all employees, the maximum salary, the

minimum salary, and the average salary.

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

AVG (SALARY)

FROM EMPLOYEE

QUERY 12:

Count the number of distinct salary values in the database.

Q.12: SELECT COUNT(ISTINCT SALARY)

FROM EMPLOYEE

Notice that, if we write COUNT(SALARY) instead of COUNT(DISTINCT

SALARY) IN Q 19, we get the same result as COUNT(*)because duplicates

will not be eliminated.


Write the syntax we use to create a table in SQL.



8.6 Summary

This unit explained concepts such as

o Data Retrieval Statement (SELECT)

o Multi table Queries

o Nested Queries Or Sub queries:

o Multiple Row Nested Queries:

o The Exists Clause

o Data Manipulation Language

8.7 Terminal Questions (TQs)

Q. 1 What is SQL? Explain.

Q. 2 Discuss data retrieval using SQL.

Q. 3 Discuss the concept of Multi-table queries.

8.8 Multiple Choice Questions (MCQs)

1. To find the names of all branches in the relation

loan = (loan no., branch_name, amount ) we use the following SQL

statement

a) select branch name from loan ;

b) Select * from loan;

c) select all branch name;

d) select branch name from loan where amount >0;

2. If account=(account_no, branch_name, balance) is a relation, then to get

average balance for each branch we use

a) Group by clause b) average clause

c) having clause d) select clause

3. Consider a relation account= (account_no, branch_name, balance). If 5%

interest is to be paid only to accounts with balances $1000 or more, the SQL

statement is:



a) Update table account set balance = balance * 5% where balance

>1000;

b) Update set balance = balance * 1.05 where balance >= 1000;

c) Update account set balance = balance * 1.5 where balance > = 1000;

d) Update balance * 1.05 for account;

4. A command used to modifying the structure of tables is…..

a) Drop b) Change

c) Modify d) Alter

8.9 Answers to SAQs, TQs, and MCQs

8.9.1 Answers to Self Assessment Questions (SAQs)

For Section 8.1

1. SQL stands for structured Query Language. It is a non-procedural

language, meaning that SQL describes what data to retrieve, delete or

insert rather than how to perform the operation. It is the standard

command set used to communicate with the RDBMS. (Refer section 8.1)

2. THE SQL STATEMENT CAN BE GROUPED INTO THE FOLLOWING

CATEGORIES.

5. DDL(Data Definition Language)

6. DML(Data Manipulation Language)

7. DCL(Data Control Language)

8. TCL(Transaction Control Language) (Refer section 8.1)

3. DCL: (Data Control Language)

The Data Control Language Statements are used to Grant permission to

the user, Revoke permission from the user and Lock certain Permission

for the user. (Refer section 8.1)

For Section 8.2

1. The select statement is used to extract information from one or more

tables in the database. (Refer section 8.2)



2.

RELATIONAL OPERATOR S AND COMPARISON CONDITIONS:

= Equal to

> Greater than

>= Greater than or equal

< Less than

<= Less than or equal

<> Not equal

BETWEEN<a>ABD<b> Range between <a> and <b> inclusive

1N<set> True when the member is in the <set>

LIKE<pattern> Matches a specified pattern

IS NULL Is a null value

(Refer section 8.2)

3. SORTING (ORDER BY CLAUSE):

It gives a result in a particular order. Select all the employees lists

sorted by name, salary in descending order.

Select* from emp order by basic;

Select job, ename from emp order by joindate desc;

Desc at the ed of the order by clause orders the list in descending order

instead of the default [ascending] order. (Refer section 8.2)

4. SQL supports range searches. For e.g. If we want to see all the

employees with salary between 22000 and 32000

SELECT * from employee

WHERE salary between 22000 and 32000

(Refer section 8.2)

5. AGGREGATE FUNCTIONS AND GROUPING:

Group by clause is used to group the rows based on certain common

criteria, for e.g. we can group the rows in an employee table by the

department. (Refer section 8.2)



For Section 8.3

1. When two tables are joined together we must follow these guidelines:

Table names in the FROM clause are separated by commas.

Use appropriate joining condition. This means that the foreign key of

table 1 will be made equal to the primary key of table 2. This column

acts as the joining attribute. For example, dno of employee table

and dno of department will be involved in the joining condition of the

WHERE clause. (Refer section 8.3)

2. A where clause generally contains a condition; but it can also contain an

SQL query. The query is within a WHERE clause is called the inner

query or sub query, and the query that encloses the inner one is called

the outer query or main query. It is also possible to place the inner

query with in a FROM or HAVING clause. Using nested queries it is

possible to build powerful SQL programs. (Refer section 8.3.1)

3. The query is within a WHERE clause is called the inner query or sub

query and the query that encloses the inner one is called the outer query

or main query. (Refer section 8.3.1)

4. The operators IN, ANY, and ALL are used in the multiple row sub

queries. The sub query in this case returns more than one row. (Refer

section 8.3.2)

5. The exits clause returns true in a WHERE clause if the subquery that

follows returns at least one row. (Refer section 8.3.3)

For Section 8.4

1. To add a new row into the table you can follow the syntax:

INSERT INTO table [(column-1I, column-2…I)]

Values (value- I I, value-2…..I);

Using this syntax you can insert only one row at a time.

(Refer section 8.4)

For Section 8.5

1. syntax: CREATE TABLE tablename

(column_1 datatype,column_2 datatype………….);



You specify the name of the table (should be unique) and one or more

attributes and their data types. (Refer section 8.5)

8.9.2 Answers to Terminal Questions (TQs)

1. SQL stands for structured Query Language, it is a non-procedural

language, meaning that SQL describes what data to retrieve, delete or

insert rather than how to perform the operation. (Refer section 8.1)

2. The select statement is used to extract information from one or more

tables in the database. (Refer section 8.2)

3. There are many occasions in the database applications where we need

to retrieve data from more than one table. (Refer section 8.3)

8.9.3 Answers to Multiple Choice Questions (MCQs)

1. A

2. A

3. C

4. D

08-Unit8

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