Question

I have a MySQL query left joining two tables. This is the current query result:

id | login | privacy-weight | requires
--------------------------------------------
0  | user  | 1              | NULL
0  | user2 | 1              | NULL
0  | user3 | 1              | privacy-weight

The query itself is not important, as I’d only like to add a WHERE condition to the query as it is now.

I need to fetch only values which (in my own words):

IF (`requires` = 'privacy-weight'), then `privacy-weight` must equal = 0;

That means, I need this condition:

WHERE `privacy-weight` = 0

BUT only if this is true:

requires = 'privacy-weight'

Can this be done?

EDIT

Obviously this is too difficult to understand, therefore, an example output:

privacy-weight | requires
-------------------------
0              | NULL
1              | NULL
0              | privacy-weight
1              | NULL

These would be ignored (not fetched):

privacy-weight | requires
-------------------------
1              | privacy-weight

Answer 1

You could use this simple test:

WHERE (requires = 'privacy-weight' AND privacy-weight = 0) OR requires <> 'privacy-weight' OR requires IS NULL

The first part (requires = 'privacy-weight' AND privacy-weight = 0) prevents the output of:

privacy-weight | requires
-------------------------
1              | privacy-weight

But keeps:

privacy-weight | requires
-------------------------
0              | privacy-weight

While the second part OR requires <> 'privacy-weight' OR requires IS NULL will keep the following ones:

privacy-weight | requires
-------------------------
0              | NULL
1              | NULL
1              | NULL