Count function
Count function operate on a group of rows and return one row of output.
You can use count function with any valid expression. For example, you can use the COUNT() function with numbers, strings, and datetimes.
Null values are ignored by count function.
You can use the DISTINCT keyword to exclude duplicate entries.
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>
SQL> select count(empno) from emp;
COUNT(EMPNO)
------------
11
SQL>
Using ROWID with Count
SQL>
SQL> select count(rowid) from emp;
COUNT(ROWID)
------------
11
SQL>
Using Count function with group by clause:
SQL>
SQL>
SQL> select count(rowid) from emp group by deptno;
COUNT(ROWID)
------------
5
4
2
SQL>