Question

I have a table with the following possible values for the column field_value.

But when I try to select all values which are not ‘CB’, the query result also ignores all NULL values.

Why does this happen?

mysql> select distinct field_value from TableName;
+--------------+
| field_value  |
+--------------+
| S            |
| NULL         |
| CA           |
| CB           |
+--------------+
4 rows in set (6.32 sec)

mysql> select distinct field_value from TableName where field_value!='CB';
+--------------+
| field_value  |
+--------------+
| S            |
| CA           |
+--------------+
2 rows in set (0.15 sec)

mysql>

Answer 1

That’s because any comparisons with NULL also yield NULL (i.e. not truthy nor falsy).

The special operators IS NULL and IS NOT NULL are used to make useful comparisons against NULL.

Your query should therefore be:

... WHERE field_value!='CB' OR field_value IS NULL;

See also: Working with NULL values