Using LIMIT 1 in MySQL

Question

When is it appropriate to add LIMIT 1 at the end of the query in MySQL. I normally add it in DELETE but I\'ve seen it being used with INSERT a and even UPDATE. Is it an overkill

Answer 1

 INSERT INTO .. VALUES () LIMIT 1

Doesn't exist. Hopefully you know how many VALUES() you put in there!

 INSERT INTO .. SELECT ... LIMIT 1

Does exist and is pretty useful, and off topic since the LIMIT is on the SELECT.

 DELETE ... LIMIT 1
 UPDATE ... LIMIT 1

Extremely rarely useful. Either you know your database enough to be certain that your WHERE matches a UNIQUE condition, or you don't, in which case you should spend a little more time looking at your database and learning SQL.

But ...

 UPDATE jobs SET owner=me WHERE owner IS NULL ORDER BY job_submit_time LIMIT 1

Can be extremely useful! This makes a near-lockless job queue, where you can come and take a job from the queue without any waiting, locking, or conflict resolution. Quite excellent.

 DELETE FROM cache ORDER BY last_update_time LIMIT N

Cache takes too much space ? Purge the N oldest rows...