仅当其他表中的字段满足条件时，MySQL UPDATE

Question

I am having a little trouble trying to accomplish this. Here is the gist of what I need to do:

UPDATE links SET
 link = '$link', rid = $rid, order = $order
WHERE lid = $lid
IF (SELECT COUNT(*) FROM resources WHERE rid = $rid AND (sid = $sid OR sid IS NULL) AND types IS NULL) == 1;

So basically, I want to run the UPDATE if and only if the resource in the resources table is associated with the site (sid) or is not associated with any particular site and types is null.

I suppose I can run a PHP conditional, but it would be preferable if I could do this with one query. Is it possible?

Thansk so much in advance!

Answer 1

UPDATE links
SET link = '$link', rid = $rid, order = $order
WHERE lid = $lid
and (SELECT COUNT(*) 
    FROM resources 
    WHERE rid = $rid 
    AND (sid = $sid OR sid IS NULL) 
    AND types IS NULL) = 1;