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I have very simple query like this:

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

And my deleted_1 be default is null or some user id, but for some reason this query always returns me 0 rows, i also tried <> but still no luck what could be wrong?

EDTI So after running more querys i find out that my problems was default value of deleted_1 field, it was NULL so i modified my query and now it works fine:

SELECT *
FROM `all_conversations`
WHERE `deleted_1` != 'NULL'
AND `deleted_1` != 23

Answers

SELECT * FROM all_conversations WHERE deleted_1 <> 1 OR deleted_1 IS NULL

NULL values need special treatment: http://dev.mysql.com/doc/refman/5.1/en/working-with-null.html

I'd suggest using the diamond operator (<>) in favor of != as the first one is valid SQL and the second one is a MySQL addition.