6
Subqueries
Objectivesj i
After completing this lesson, you should be able to do the following:
•• Describeri thet typest off problemsr l thatt t subqueriesri can solvel
•• Definefi subqueriesri
•• Listi t thet typest off subqueriesri
•• Writerite singlei le--rowr and multiplelti le--rowr subqueriesri
6-2
Usingi a Subquery to Solvel a Probleml
“Who has a salaryl greater than Jones’s?”’
Main Query
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“Which employees have a salary greater |
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than Jones’s salary?” |
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Subquery |
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“What is Jones’s salary?” |
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6-3
Subqueriesi
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SELECT |
select_list |
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FROM |
table |
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WHERE |
expr operator |
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(SELECT |
select_list |
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FROM |
table); |
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•• TheThesubquerysubquery(inner(innerquery)query)executesexecutesonceoncebeforebeforethethemainainqueryquery..
•• TheTheresultresultofofthethesubquerysubqueryisisusedusedbybythethemainainqueryquery(outer(outer query)query)..
6-4
Usingi a Subquery
SQL> |
SELECT |
ename |
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2 |
FROM |
emp |
2975 |
3 |
WHERE |
sal |
> |
4 |
(SELECT sal |
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FROM |
emp |
6 |
WHERE |
empno=7566); |
ENAME
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KING
FORD
SCOTT
6-5
Guidelinesi li for Usingi Subqueriesi
•• Enclosel subqueriesri inin parenthesesr t ..
•• Placel subqueriesri on thet rightri t sidei off thet comparisonri operatorr t r..
•• Do nott add an ORDER BY clausel toto a subqueryry..
•• Use singlei le--rowr operatorsr t rs withith singlei le-- rowr subqueriesri ..
•• Use multiplelti le--rowr operatorsr t rs withith multiplelti le--rowr subqueriesri ..
6-6
Types of Subqueriesi
•• Singlei le--rowr subqueryry
Main query
returns
Subquery
•• Multiplelti le--rowr subqueryry
Main query
returns
Subquery
•• Multiplelti le--columnl subqueryry
Main query
returns
Subquery
6-7
CLERK
CLERK
MANAGER
CLERK 7900 MANAGER 7698
Singlei le-Row Subqueriesi
•• Returnt rn onlyly one rowr
•• Use singlei le--rowr comparisonri operatorsr t rs
Operator Meaning
=Equal to
>Greater than
>= |
Greater than or equal to |
<Less than
<= |
Less than or equal to |
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<> |
Not equal to |
6-8
Executingi Singlei le-Row Subqueriesi
SQL> |
SELECT |
ename, job |
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2 |
FROM |
emp |
CLERK |
3 |
WHERE |
job = |
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(SELECT |
job |
5 |
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FROM |
emp |
6 |
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WHERE |
empno = 7369) |
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AND |
sal > |
1100 |
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(SELECT |
sal |
9 |
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FROM |
emp |
10 |
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WHERE |
empno = 7876); |
ENAME JOB
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MILLER CLERK
6-9
Usingi Group Functionsi
inin a Subquery
SQL> |
SELECT |
ename, job, sal |
800 |
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2 |
FROM |
emp |
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3 |
WHERE |
sal = |
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(SELECT |
MIN(sal) |
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5 |
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FROM |
emp); |
ENAME |
JOB |
SAL |
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--------- --------- |
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SMITH |
CLERK |
800 |
6-10
HAVINGI Clausel withi Subqueriesi
•• The Oracler le Serverr r executest subqueriesri firstfir t..
•• The Oracler le Serverr r returnsr t r resultsr lts intoi to thet mainin query’sr ’s HAVINGI clausel ..
SQL> |
SELECT |
deptno, MIN(sal) |
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2 |
FROM |
emp |
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3 |
GROUP BY |
deptno |
800 |
4 |
HAVING |
MIN(sal) > |
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(SELECT |
MIN(sal) |
6 |
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FROM |
emp |
7 |
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WHERE |
deptno = 20); |
6-11