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17424570
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CreationDate
2013-07-02T11:36:49.113
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2014-06-03T21:34:43.247
LastEditDate
2017-05-23T12:18:12.490
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79202
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17406675
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2
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11
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text
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It cannot be done without a stored procedure or function, unfortunately. I figured out how to support a function in my app, though. Borrowing the basic procedure idea from <a href="https://stackoverflow.com/a/16644480/79202">this answer</a>, I've come up with this: <pre><code>DELIMITER | CREATE FUNCTION checkit(doit INTEGER, message VARCHAR(256)) RETURNS INTEGER DETERMINISTIC BEGIN IF doit IS NULL OR doit = 0 THEN SIGNAL SQLSTATE 'ERR0R' SET MESSAGE_TEXT = message; END IF; RETURN doit; END; | </code></pre> The idea is that the function can be used in triggers like a <code>CHECK</code> constraint, or inline in SQL statements. Getting back to my original need to throw an error if a user does not exist, I now use the <code>checkit()</code> function like this: <pre><code>SELECT checkit(COUNT(*), 'User "foo" does not exist') FROM mysql.user WHERE user = 'foo'; </code></pre> If user <code>foo</code> exists, this query returns an integer. If the user does not exist, it throws an error with the message defined there. Want to use the function for a check constraint, too? He's an example (mimicking <a href="https://stackoverflow.com/a/9737269/79202">this answer</a>), with a tip of the hat to @rouland-bouman: <pre><code>CREATE TRIGGER mytabletriggerexample BEFORE INSERT FOR EACH ROW BEGIN SET @dummy := checkit( NEW.important_value) >= (fancy * dancy * calculation), 'Your meaningful error message goes here' ); END; </code></pre> I would rather use <code>DO</code>, rather than setting a dummy variable, but <a href="http://bugs.mysql.com/bug.php?id=69647" rel="nofollow noreferrer">a MySQL bug</a> prevents that from working, alas.
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1. POHow do I get MySQL to throw a conditional runtime exception in SQL
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