Question

I have a simple table on my local server.

mysql> desc table ;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(10) | YES  |     | NULL    |       | 
| count | int(10) | YES  |     | NULL    |       | 
+-------+---------+------+-----+---------+-------+
2 rows in set (0.00 sec

It has just three records in it.

mysql> select * from uday ;
+------+-------+
| id   | count |
+------+-------+
|    1 |     1 | 
|    2 |     2 | 
|    3 |     0 | 
|    4 |  NULL | 
+------+-------+
4 rows in set (0.00 sec)

Now, why is that i am not seeing the fourth column in the below result..?

mysql> select * from uday where count NOT IN (0) ;
mysql> select * from uday where count != 0 ;
+------+-------+
| id   | count |
+------+-------+
|    1 |     1 | 
|    2 |     2 | 
+------+-------+
2 rows in set (0.00 sec)

how about the 4th record…? Its not visible in the result.
NULL is not 0 RIGHT…?

please ignore if it looks pretty foolish as I am not even competitive in coding part.

Answer 1

col1 not in (1,2,null)

is shorthand for:

col1 <> 1 and col1 <> 2 and col1 <> null

In SQL’s three-valued logic, col1 <> null returns unknown. And true and true and unknown also returns unknown. Since where only accepts true, and not unknown, the row with null is filtered out of the result set.