testing null

 mysql> SELECT product_id,if(isnull(price)=0,'A','B') as p from products;

---------

mysql> SELECT product_id,isnull(price) as p from products;

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

| product_id | p |

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

|          1 | 0 |

|          2 | 0 |

|          3 | 0 |

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

3 rows in set (0.00 sec)

mysql> SELECT IF(NULL = NULL, 1, 3);

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

| IF(NULL = NULL, 1, 3) |

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

|                     3 |

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

1 row in set (0.00 sec)

mysql> SELECT IF(NULL is NULL, 1, 3);

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

| IF(NULL is NULL, 1, 3) |

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

|                      1 |

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