SQL NULL Example :

select ename, sal, job, case when job ='CLERK' then '15%' when JOB='SALESMAN' then '20%' when ename= 'SMITH' then '25%' end as raise from emp where comm is not null

In this query , "SMITH' won't be printed, as the clause "NOT NULL, would mask the column originally, NULL.

Please Also review the example with NULL

Note for any conflicting or competing clauses.
EMP Table:

Oracle:

In the output SMITH , row was dropped.

 
PostgreSQL

In the out put the row "SMITH" was not printed.