query “not equal” doesn't work

Question

I have very simple query like this:

SELECT * FROM `all_conversations` WHERE `deleted_1` != \'1\';

And my deleted_1 be default

Answer 1

You cannot use arithmetic comparison operators such as =, <, or <> to test for NULL.

To demonstrate this for yourself, try the following query:

mysql> SELECT 1 = NULL, 1 <> NULL, 1 < NULL, 1 > NULL;
+----------+-----------+----------+----------+
| 1 = NULL | 1 <> NULL | 1 < NULL | 1 > NULL |
+----------+-----------+----------+----------+
|     NULL |      NULL |     NULL |     NULL |
+----------+-----------+----------+----------+

so you have to say,

SELECT * FROM `all_conversations` WHERE `deleted_1` <> '1' and `deleted_1` is null;

your_field IS NULL explicitly since NULL cant be club with values using arithmetic operators and it has own value BOOLEAN