sql - Why does column = NULL return no rows? -

possible duplicate:
why null = null evaluate false in sql server

if generate query insert data in table "mytab" column --- age, sex, dob, id

insert mytab  values (22, '', '', 4)

what'll value in column sex & dob ? null ?

if value null ---

 select * mytab  sex=null

above query gives output ---- no rows selected --- why ??

if value not null ---

 select * mytab  sex null

above query gives output ---- how ??

null special value in sql denoting absence of data. such, cannot queries like:

select fields table column = null

null cannot compared anything, including null itself. instead, you'd need:

select fields table column null

however in case inserting empty value in sex , dob.
, empty value not null. you'd have query for:

select fields table column = ''

ERT