Week 2a Date

8/9/2019 Week 2a Date

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ObjectivesAfter completing this lesson, you should be able to do the following:

Describe the types of problem that subqueries can solve

Define subqueries

List the types of subqueries

Write single-row and multiple-row subqueries


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Using a Subquery to Solve a Problem

Which employees have salaries

greater than Turner salary?

What is Turner salary?

Who has a salary greater than Turners ?

Suppose you want to write a query to find out who earns a salary greater than

Turners salary.

To solve this problem, you need 2 queries.

One to find what Turner earns, andSecond query to find who earns more than that amount.

You can solve this problem by combining the 2 queries,placing one query

INSIDE the other query.

The inner query, also called the SUBQUERY, returns a value that is used byMAIN QUERY.


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SELECT ENAME,SAL FROM EMP

WHERE

SAL> (SELECT SAL FROM EMP WHERE ENAME='TURNER')


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SUBQUERY SYNTAX

SELECT select_list

FROM table

WHERE operator(SELECT select_list FROM table ) ;

Subquery includes

WHERE clause

HAVING clause

FROM clause

Operator includes

Single row : = < >=

Multiple row : IN ANY ALL


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TYPES OF SUBQUERY

Main Query

Subquery FORD

Main Query

SubqueryFORD

TURNER

SCOTT

Single-Row subquery

Multiple-Row subquery

returns

returns

Return only

one row from

the inner

select

statement.

Return more

than one row

from the

inner select

statement.


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SINGLE - ROW SUBQUERIES

Operator Meaning

= Equal to

> Greater than

>= Greater than or equal to

< Less than


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SELECT empno,ename,job

FROM empWHERE job=

( SELECT job FROM emp

WHERE empno=7521 );

Display the employees whose job is the same as that employee 7521.


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SELECT empno,ename,job,sal

FROM empWHERE job=

( SELECT job FROM emp

WHERE empno=7521 )

AND sal>( SELECT salFROM emp

WHERE empno=7654 )

Display the employees whose job is the same as that employee 7521 and

Whose salary is greater than that employee 7654.

>1250 TRUE

>1250 FALSE

>1250 FALSE

>1250 TRUE


10/29

SELECT empno,ename, job,sal

FROM emp

WHERE sal=

( SELECT min(sal) FROM emp )

Using Group Function in a Subquery

Displays the employee number, name, job and salary of all employees whose

salary is equal to the minimum salary.


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The HAVING clause with subqueries

SELECT deptno,min(sal)

FROM emp

GROUP BY deptnoHAVING min(sal) >

(SELECT min(sal)

FROM emp

WHERE deptno=20 )

Displays all departments that have a minimum salary greater than that of

department 20.


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SELECT job,avg(sal)

FROM emp

GROUP BY jobHAVING avg(sal) =

(SELECT min(avg(sal))

FROM emp

GROUP BY job )

Find the job with the lowest average salary.


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SELECT empno,ename,sal

FROM emp

WHERE sal =

ERROR at line 3 :ORA-01427: Single-row subquery returns more than one row.

What is Wrong with This Statement ?

( SELECT MIN(sal)FROM emp

GROUP BY deptno )

One common error with subqueries is more than one row returned for single-row subquery.

Subquery contain GROUP BY clause, which implies that the subquery will return multiple rows,

One for each group it finds.In this case , the results of subquery will be 950, 800, and 1300.

The main query takes the result of subquery (950,800,1300) and uses these results in its WHERE

clause.

WHERE clause contain an uqual = operator, a single-row comparison operator expecting only

one value.


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FROM emp

WHERE sal in

How Can You Correct This Statement ?

( SELECT MIN(sal)FROM emp

GROUP BY deptno )

To correct this error , change the = operator to IN


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Multiple-Row Subquery

Operator Meaning

IN Equal to any member in the list

ANY Compare value to each value returned by the subquery.

ALL Compare value to every value returned by the subquery.

Returns more than one row

Use multiple row comparison operators.

Fi d h l h h l h i i l f h


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FROM empWHERE sal in

( SELECT MIN(sal)

FROM emp

GROUP BY deptno )

Find the employees who earn the same salary as the minimum salary for each

departments.

( 950 , 800 , 1300 )

The inner query executed first,

producing a query result.Main query uses that results.

In fact, the main query would look

like the following.


FROM empWHERE sal in (950,800,1300)

U i th ANY O t i M lti l R S b i


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Using the ANY Operator in Multiple-Row Subqueries


FROM emp

WHERE job SALESMAN

AND

sal < ANY ( SELECT sal

FROM emp

WHERE job=SALESMAN

The ANY operator compares a value to each value returned by a subquery.

Find employees who are not SALESMAN and whose salary is less than

that of any SALESMAN

< ANY means less than the maximum > ANY means more than the minimum


FROM emp

WHERE jobSALESMAN

AND

sal < ANY ( SELECT sal FROM emp

WHERE job=SALESMAN )

= ANY is equivalent to IN


FROM emp

WHERE jobSALESMAN

AND

sal < ( SELECT MAX(sal) FROM emp


SAME


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SAL


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Using the ALL Operator in Multiple-Row Subqueries


FROM emp

WHERE job SALESMAN

AND

sal < ALL ( SELECT sal

FROM emp

WHERE job=SALESMAN

The ANY operator compares a value to everyvalue returned by a subquery.

Find employees who are not SALESMAN and whose salary is less than

that of all SALESMAN

< ALL means less than the minimum > ALL means more than the maximum


FROM emp

WHERE jobSALESMAN

AND

sal < ANY ( SELECT sal FROM emp


The NOT operator can be used with IN, ANY and ALL operators


FROM emp

WHERE jobSALESMAN

AND

sal < ( SELECT MAX(sal) FROM emp


SAME


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SAL


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FROM emp

WHERE job SALESMAN AND

SAL < ANY ( SELECT sal FROM emp

WHERE job=SALESMAN


FROM emp

WHERE job SALESMAN AND

SAL < ALL ( SELECT sal FROM emp

WHERE job=SALESMAN

SALESMANS SALARIES1250

1500

1600

1400

ANY ALL

1300 is less than all but not

from 1250.


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SELECT ENAME,SAL FROM EMP

WHERE SAL< ANY(SELECT MIN(SAL) FROM EMP GROUP BY DEPTNO)

EMP

1300

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