Correlated subqueries

Correlated subqueries reference one or more columns in the outer SQL statement.

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, sal
  2  FROM emp outer
  3  WHERE sal = (SELECT AVG(sal) FROM emp inner WHERE inner.empno = outer.empno);

     EMPNO ENAME             SAL
---------- ---------- ----------
         1 SMITH             800
         2 ALLEN            1600
         3 WARD             1250
         4 JONES            2975
         5 MARTIN           1250
         6 BLAKE            2850
         7 CLARK            2850
         8 SCOTT            3000
         9 KING             3000
        10 TURNER           1500
        11 ADAMS            1500

11 rows selected.

SQL>