Try out following examples:

root@host# mysql -u root -p password;

Enter password:*******

mysql> use TUTORIALS;

Database changed

mysql> create table tcount_tbl

-> (

-> tutorial_author varchar(40) NOT NULL,

-> tutorial_count INT

-> );

Query OK, 0 rows affected (0.05 sec)

mysql> INSERT INTO tcount_tbl

-> (tutorial_author, tutorial_count) values ('mahran', 20);

mysql> INSERT INTO tcount_tbl

-> (tutorial_author, tutorial_count) values ('mahnaz', NULL);

mysql> INSERT INTO tcount_tbl

-> (tutorial_author, tutorial_count) values ('Jen', NULL);

mysql> INSERT INTO tcount_tbl

-> (tutorial_author, tutorial_count) values ('Gill', 20);

mysql> SELECT * from tcount_tbl;

+-----------------+----------------+

| tutorial_author | tutorial_count |

+-----------------+----------------+

| mahran | 20 |

| mahnaz | NULL |

| Jen | NULL |

| Gill | 20 |

+-----------------+----------------+

4 rows in set (0.00 sec)

mysql>

You can see that = and != do not work with NULL values as follows:

mysql> SELECT * FROM tcount_tbl WHERE tutorial_count = NULL;

Empty set (0.00 sec)

mysql> SELECT * FROM tcount_tbl WHERE tutorial_count != NULL;

Empty set (0.01 sec)

To find records where the tutorial_count column is or is not NULL, the queries should be written like this:

mysql> SELECT * FROM tcount_tbl

-> WHERE tutorial_count IS NULL;

+-----------------+----------------+

| tutorial_author | tutorial_count |

+-----------------+----------------+

| mahnaz | NULL |

| Jen | NULL |

+-----------------+----------------+

2 rows in set (0.00 sec)

mysql> SELECT * from tcount_tbl

-> WHERE tutorial_count IS NOT NULL;

+-----------------+----------------+

| tutorial_author | tutorial_count |

+-----------------+----------------+

| mahran | 20 |

| Gill | 20 |

+-----------------+----------------+

2 rows in set (0.00 sec)