Use subqueries in table join

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 e.deptno, sal, empSum.emp_count
  2  FROM emp e,
  3      (SELECT deptno, COUNT(empno) emp_count FROM emp GROUP BY deptno) empSum
  4  WHERE e.deptno = empSum.deptno;

    DEPTNO        SAL  EMP_COUNT
---------- ---------- ----------
        30       1500          5
        30       2850          5
        30       1250          5
        30       1250          5
        30       1600          5
        20       1500          4
        20       3000          4
        20       2975          4
        20        800          4
        10       3000          2
        10       2850          2

11 rows selected.

SQL>