An INSERT conditioned on COUNT

Posted by Anders Feder on Stack Overflow See other posts from Stack Overflow or by Anders Feder
Published on 2010-12-28T02:36:38Z Indexed on 2010/12/28 2:54 UTC
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sql

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mysql

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insert

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count

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conditional

How can I construct a MySQL INSERT query that only executes if the number of rows satisfying some condition already in the table is less than 20, and fails otherwise?

That is, if the table has 18 rows satisfying the condition, then the INSERT should proceed. If the table has 23 rows satisfying the condition, then the INSERT should fail.

For atomicity, I need to express this in a single query, so two requests can not INSERT at the same time, each in the 'belief' that only 19 rows satisfy the condition.

Thank you.

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