IS NULL operator has only one operand: the preceding column name or column expression.
Consider the following code:
select * from emp where comm IS null select * from emp where comm = null
If one or both of the operands being compared by the equality comparison operator = evaluates to a null value, the result is UNKNOWN.
You cannot say that a null value is equal to a null value.
The following shows the conclusions:
| Expression | Evaluates to |
|---|---|
| NULL = NULL | UNKNOWN |
| NULL IS NULL | TRUE |
This explains why the following query doesn't return all 14 rows of the EMPLOYEES table.
select ename, init from emp where comm = comm;
SQL> SQL> drop table emp; Table dropped.-- from w w w.j av a2s . c om SQL> create table emp( 2 empno NUMBER(4) primary key, 3 ename VARCHAR2(8) not null , 4 init VARCHAR2(5) not null , 5 job VARCHAR2(8) , 6 mgr NUMBER(4) , 7 bdate DATE not null , 8 msal NUMBER(6,2) not null , 9 comm NUMBER(6,2) , 10 deptno NUMBER(2) default 10) ; SQL> SQL> insert into emp values(7001,'SMITH','N', 'TRAINER', 7902,date '1975-12-17', 1800 , NULL, 20); SQL> insert into emp values(7002,'ALLEN','JAM','SALESREP',7006,date '1971-05-20', 1600, 300, 30); SQL> insert into emp values(7003,'WARD', 'TF' ,'SALESREP',7006,date '1972-03-02', 1250, 500, 10); SQL> insert into emp values(7004,'JACK', 'JM', 'MANAGER', 7009,date '1977-04-02', 2975, NULL, 20); SQL> insert into emp values(7005,'BROWN','P', 'SALESREP',7006,date '1976-09-28', 1250, 1400, 30); SQL> insert into emp values(7006,'BLAKE','R', 'MANAGER', 7009,date '1973-11-01', 2850, NULL, 10); SQL> insert into emp values(7007,'CLARK','AB', 'MANAGER', 7009,date '1975-06-09', 2450, NULL, 10); SQL> insert into emp values(7008,'SCOTT','DEF','TRAINER', 7004,date '1979-11-26', 3000, NULL, 20); SQL> insert into emp values(7009,'KING', 'CC', 'DIRECTOR',NULL,date '1972-10-17', 5000, NULL, 10); SQL> insert into emp values(7010,'BREAD','JJ', 'SALESREP',7006,date '1978-09-28', 1500, 0, 30); SQL> insert into emp values(7011,'ADAMS','AA', 'TRAINER', 7008,date '1976-12-30', 1100, NULL, 20); SQL> insert into emp values(7012,'JONES','R', 'ADMIN', 7006,date '1979-10-03', 8000, NULL, 30); SQL> insert into emp values(7902,'FORD', 'MG', 'TRAINER', 7004,date '1979-02-13', 3000, NULL, 20); SQL> insert into emp values(7934,'MARY', 'ABC','ADMIN', 7007,date '1972-01-23', 1300, NULL, 10); SQL> SQL> SQL> select * from emp where comm IS null; EMPNO | ENAME | INIT | JOB | MGR | BDATE | MSAL --------- | -------- | ----- | -------- | --------- | --------- | --------- COMM | DEPTNO --------- | --------- 07001.00 | SMITH | N | TRAINER | 07902.00 | 17-DEC-75 | 01800.00 [N/A] | 00020.00 07004.00 | JACK | JM | MANAGER | 07009.00 | 02-APR-77 | 02975.00 [N/A] | 00020.00 07006.00 | BLAKE | R | MANAGER | 07009.00 | 01-NOV-73 | 02850.00 [N/A] | 00010.00 EMPNO | ENAME | INIT | JOB | MGR | BDATE | MSAL --------- | -------- | ----- | -------- | --------- | --------- | --------- COMM | DEPTNO --------- | --------- 07007.00 | CLARK | AB | MANAGER | 07009.00 | 09-JUN-75 | 02450.00 [N/A] | 00010.00 07008.00 | SCOTT | DEF | TRAINER | 07004.00 | 26-NOV-79 | 03000.00 [N/A] | 00020.00 07009.00 | KING | CC | DIRECTOR | [N/A] | 17-OCT-72 | 05000.00 [N/A] | 00010.00 EMPNO | ENAME | INIT | JOB | MGR | BDATE | MSAL --------- | -------- | ----- | -------- | --------- | --------- | --------- COMM | DEPTNO --------- | --------- 07011.00 | ADAMS | AA | TRAINER | 07008.00 | 30-DEC-76 | 01100.00 [N/A] | 00020.00 07012.00 | JONES | R | ADMIN | 07006.00 | 03-OCT-79 | 08000.00 [N/A] | 00030.00 07902.00 | FORD | MG | TRAINER | 07004.00 | 13-FEB-79 | 03000.00 [N/A] | 00020.00 EMPNO | ENAME | INIT | JOB | MGR | BDATE | MSAL --------- | -------- | ----- | -------- | --------- | --------- | --------- COMM | DEPTNO --------- | --------- 07934.00 | MARY | ABC | ADMIN | 07007.00 | 23-JAN-72 | 01300.00 [N/A] | 00010.00 10 rows selected. SQL> select * from emp where comm = null; no rows selected SQL> select ename, init 2 from emp 3 where comm = comm; ENAME | INIT -------- | ----- ALLEN | JAM WARD | TF BROWN | P BREAD | JJ SQL>