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

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

There are instances where a limit 1 is REALLY useful. For example, a table without a unique key. If we have five of the exact same, then limit one will work.

Answer 2

Well, if you use EXPLAIN you'll see that it speeds things up, as once it's found one result, it stops.

It's also a failsafe – if you know your insert of update should only ever affect one row, by specifying it, you are guaranteeing that it won't ever go wrong and mess up multiple rows.

Answer 3

I can't think of an instance where something that speeds up a query and makes it more secure at the same time could be called overkill. I'll put my vote in the good practice column.

Answer 4

Assume you are writing Forgot Password code (in PHP):

UPDATE
    `users`
SET
    `pass` = PASSWORD('{$new_salted_pass}')
WHERE
    `account_id` = '{$account_id}'
LIMIT 1

If your software is a Software as a Service (SaaS),

and multiple organizations use your software,

and account_id-organization_id fields together are a unique id,

and the account_id is auto-generated in the code like so:

JONES0001
JONES0002
JONES0003
...

and the software engineer forgets to add AND organization_id = '{$organization_id}' to the query's WHERE condition,

and the code passes QA when testing the Forgot Password functionality in QA environment,

and the code is then pushed to production,

then the LIMIT 1 can limit the damage done when two organizations' user's share the same account_id.

Answer 5

 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...