如果MySQL同时获得相同的UPDATE查询会发生什么？

Question

I have a query like this:

UPDATE persons SET processing = :processing WHERE processing IS NULL LIMIT 1

What if the script gets called twice at the exact time to execute this query? Is it possible that it just updates the same row twice?

Answer 1

It will execute it twice, atomically. If processing is changed to not null, then the second operation in the queue will do nothing.

If you are thinking about the 'rows_affected' value, it will say 0 rows affected the second time, that is unless you change a connection parameter that tells it to return any rows found rather than actually modified.

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Re-interpreting your question, if you have a table of multiple processing entries, with multiple being NULL, then no it will not update the same one, two of them will be set.