Using EXISTS and NOT EXISTS with a Correlated Subquery

EXISTS operator checks for the existence of rows returned by a subquery.


CREATE TABLE EMP (EMPNO NUMBER(4) NOT NULL,
                  ENAME VARCHAR2(10),
                  JOB VARCHAR2(9),
                  SAL NUMBER(7, 2),
                  DEPTNO NUMBER(2));

INSERT INTO EMP VALUES (1, 'SMITH', 'CLERK',     800,    20);
INSERT INTO EMP VALUES (2, 'ALLEN', 'SALESMAN', 1600,    30);
INSERT INTO EMP VALUES (3, 'WARD',  'SALESMAN', 1250,    30);
INSERT INTO EMP VALUES (4, 'JONES', 'MANAGER',  2975,    20);
INSERT INTO EMP VALUES (5, 'MARTIN','SALESMAN', 1250,    30);
INSERT INTO EMP VALUES (6, 'BLAKE', 'MANAGER',  2850,    30);
INSERT INTO EMP VALUES (7, 'CLARK', 'MANAGER',  2850,    10);
INSERT INTO EMP VALUES (8, 'SCOTT', 'ANALYST',  3000,    20);
INSERT INTO EMP VALUES (9, 'KING',  'PRESIDENT',3000,    10);
INSERT INTO EMP VALUES (10,'TURNER','SALESMAN', 1500,    30);
INSERT INTO EMP VALUES (11,'ADAMS', 'CLERK',    1500,    20);

SQL> SELECT empno, ename
  2  FROM emp outer
  3  WHERE EXISTS
  4          (SELECT empno FROM emp inner WHERE inner.deptno = outer.deptno);

     EMPNO ENAME
---------- ----------
         1 SMITH
         2 ALLEN
         3 WARD
         4 JONES
         5 MARTIN
         6 BLAKE
         7 CLARK
         8 SCOTT
         9 KING
        10 TURNER
        11 ADAMS

11 rows selected.

SQL>
SQL>

A subquery doesn't have to return a column. It can just return a literal value. This can improve the performance.


SQL> SELECT empno, ename
  2  FROM emp outer
  3  WHERE EXISTS
  4          (SELECT 1 FROM emp inner WHERE inner.deptno = outer.deptno);

     EMPNO ENAME
---------- ----------
         1 SMITH
         2 ALLEN
         3 WARD
         4 JONES
         5 MARTIN
         6 BLAKE
         7 CLARK
         8 SCOTT
         9 KING
        10 TURNER
        11 ADAMS

11 rows selected.

SQL>
SQL>

Using NOT EXISTS with a Correlated Subquery


SQL>
SQL> SELECT empno, ename
  2  FROM emp outer
  3  WHERE NOT EXISTS
  4          (SELECT empno FROM emp inner WHERE inner.deptno = outer.deptno);

no rows selected

SQL>

EXISTS checks just for the existence of rows, whereas IN checks for actual values. When a list of values contains a null value, NOT EXISTS returns true, but NOT IN returns false. NVL() can be used used to convert null values to 0:

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