Oracle Subquery
Examples and syntax explanation
Always test these 'oracle subquery' commands on a test server
before implementing them on a production server.
before implementing them on a production server.
Contents
- Definition
- Simple subquery
- Correlated subquery
- Update with correlated subquery
- Delete with correlated subquery
Definition
A subquery is basically a select clause which is used instead of another statement.
For example : Have a look at the following 2 tables : DEPT and EMP
SQL> select * from dept order by deptno;
DEPTNO DNAME LOC
---------- -------------- -------------
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
SQL> select empno,ename,job,sal,deptno from emp;
EMPNO ENAME JOB SAL DEPTNO
---------- ---------- --------- ---------- ----------
7369 SMITH CLERK 800 20
7499 ALLEN SALESMAN 1600 30
7521 WARD SALESMAN 1250 30
7566 JONES MANAGER 2975 20
7654 MARTIN SALESMAN 1250 30
7698 BLAKE MANAGER 2850 30
7782 CLARK MANAGER 2450 10
7788 SCOTT ANALYST 3000 20
7839 KING PRESIDENT 5000 10
7844 TURNER SALESMAN 1500 30
7876 ADAMS CLERK 1100 20
7900 JAMES CLERK 950 30
7902 FORD ANALYST 3000 20
7934 MILLER CLERK 1300 10
14 rows selected.
Simple Oracle subquery
A simple subquery is evaluated once for each table.
You would like to select all employees whose department is located in Chicago.
A join would be a better solution for this select,
but for the purposes of illustration we will use a subquery.
SQL> select empno,ename,job,sal,deptno from emp where
deptno in (select deptno from dept
where loc = 'CHICAGO');
EMPNO ENAME JOB SAL DEPTNO
---------- ---------- --------- ---------- ----------
7900 JAMES CLERK 950 30
7844 TURNER SALESMAN 1500 30
7698 BLAKE MANAGER 2850 30
7654 MARTIN SALESMAN 1250 30
7521 WARD SALESMAN 1250 30
7499 ALLEN SALESMAN 1600 30
6 rows selected.
You can use a subquery in the place of a table name in the 'from' clause.
SQL> Select ename, deptno from
(select ename, deptno from emp
where job = 'CLERK' )
where
deptno > 20;
ENAME DEPTNO
---------- ----------
JAMES 30
You can even replace a column name with a subquery:
SQL> select ename,
(select max(sal) from emp) "maxsal" ,
sal,
((select max(sal) from emp ) - sal )
"difference" from emp;
ENAME maxsal SAL difference
---------- ---------- ---------- ----------
SMITH 5000 800 4200
ALLEN 5000 1600 3400
WARD 5000 1250 3750
JONES 5000 2975 2025
MARTIN 5000 1250 3750
BLAKE 5000 2850 2150
CLARK 5000 2450 2550
SCOTT 5000 3000 2000
KING 5000 5000 0
TURNER 5000 1500 3500
ADAMS 5000 1100 3900
JAMES 5000 950 4050
FORD 5000 3000 2000
MILLER 5000 1300 3700
14 rows selected.
Correlated Oracle subquery
A correlated Oracle subquery is evaluated once FOR EACH ROW as opposed to a normal subquery which is evaluated only once for each table.
You can reference the outer query inside the correlated subquery using an alias which makes it so handy to use.
Let's select all employees whose salary is less than the average of all the employees' salaries in the same department.
SQL> select ename ,sal ,deptno from emp a where
a.sal < (select avg(sal) from emp b
where a.deptno = b.deptno)
order by deptno;
ENAME SAL DEPTNO
---------- ---------- ----------
CLARK 2450 10
MILLER 1300 10
SMITH 800 20
ADAMS 1100 20
WARD 1250 30
MARTIN 1250 30
TURNER 1500 30
JAMES 950 30
8 rows selected.
Using a correlated subquery in an update
Let's give these people (whose salary is less than their department's average) a raise.
SQL> select ename, sal, deptno from emp
order by deptno, ename;
ENAME SAL DEPTNO
---------- ---------- ----------
CLARK 2450 10
KING 5000 10
MILLER 1300 10
ADAMS 1100 20
FORD 3000 20
JONES 2975 20
SCOTT 3000 20
SMITH 800 20
ALLEN 1600 30
BLAKE 2850 30
JAMES 950 30
MARTIN 1250 30
TURNER 1500 30
WARD 1250 30
14 rows selected.
SQL> UPDATE emp a
set sal = (select avg(sal)
from emp b
where
a.deptno = b.deptno)
where sal <
(select avg(sal) from emp c
where a.deptno = c.deptno);
8 rows updated.
SQL> select ename, sal, deptno from emp
order by deptno, ename;
ENAME SAL DEPTNO
---------- ---------- ----------
CLARK 2916.67 10
KING 5000 10
MILLER 2916.67 10
ADAMS 2175 20
FORD 3000 20
JONES 2975 20
SCOTT 3000 20
SMITH 2175 20
ALLEN 1600 30
BLAKE 2850 30
JAMES 1566.67 30
MARTIN 1566.67 30
TURNER 1566.67 30
WARD 1566.67 30
14 rows selected.
SQL> commit;
Commit complete.
Using a correlated subquery in a delete
Let's delete the highest earning employees in each department.
SQL> delete from emp a where
a.sal = (select max(sal) from emp b
where a.deptno = b.deptno);
4 rows deleted.
SQL> select ename, sal, deptno from emp
order by deptno, ename;
ENAME SAL DEPTNO
---------- ---------- ----------
CLARK 2916.67 10
MILLER 2916.67 10
ADAMS 2175 20
JONES 2975 20
SMITH 2175 20
ALLEN 1600 30
JAMES 1566.67 30
MARTIN 1566.67 30
TURNER 1566.67 30
WARD 1566.67 30
10 rows selected.
SQL> commit;
Commit complete.
Lastly: do not forget to commit to make your changes permanent when using the oracle subquery statement.
