In Oracle NULL means absence of data. A NULL can be assigned but it cannot be equated with anything. NULL cannot be equated with NULL itself. NULL is one of the confusing feature in Oracle.
Consider below table SALARY with two columns NO and SAL. In 3rd and 4th rows SAL is NULL.
1) List all records with SAL is NULL
select * from salary where sal is null;
Note :- In SQL it will represent as is null;
2) List all records with SAL is not NULL
select * from salary where sal is not null;
3) Insert statement with NULL
insert into salary(no,sal) values (6,null);
4) Deleting all records with SAL as NULL
delete from salary where sal is null;
5) Update statement with NULL
update salary sal = null where no = 5;
6) NULL cannot be equated with NULL itself
select * from salary where null = null;
no rows selected