What happens if both values in an or statement are non-null?
Does mysql uses the left side every time or is there a possibility that mysql uses the right side?
SELECT *
FROM posts p
WHERE parent_id = ? OR id = ?
ORDER BY date
DESC LIMIT 1
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user2429266
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Of course will mysql use the right side. Why not? Do you want the check of the id only when there's no matching parent_id at all? – VMai May 09 '14 at 20:25
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1possible duplicate of [Short-circuit logic evaluation operators](http://stackoverflow.com/questions/16969780/short-circuit-logic-evaluation-operators) – Marcus Adams May 09 '14 at 20:42
1 Answers
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- If the values are
nullthen the comparison with=will fail. UseISinstead. - You can't really tell which condition will be executed first and how the DB handles the filtering. This is totally up to the optimizer at runtime.
juergen d
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I created a query that uses COALESCE( p.parent_id, p.id ). If the paren_id is NULL it will use the primary id. – user2429266 May 09 '14 at 20:56