Why does column = NULL return no rows? [duplicate]

Question

Possible Duplicate:
Why does NULL = NULL evaluate to false in SQL server

If you generate a query to insert the data in table "MyTab" for column --- Age, Sex, DOB, ID

INSERT INTO MyTab 
VALUES (22, '', '', 4)

What'll be the value in column Sex & DOB ? Is it NULL ?

If value is NULL then ---

 SELECT * FROM MyTab
 WHERE Sex=NULL

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

if value is not NULL then ---

 SELECT * FROM Mytab
 WHERE Sex IS NULL

above query gives the output ---- how ??

Answer 1

NULL is a special value in SQL denoting the absence of data. As such, you cannot do queries like:

SELECT fields FROM table WHERE column = NULL

NULL cannot be compared to anything, including NULL itself. Instead, you'd need:

SELECT fields FROM table WHERE column IS NULL

---

However in your case you are really inserting an empty value in Sex and DOB.
And empty value is not NULL. You'd have to query for:

SELECT fields FROM table WHERE column = ''