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POAre database triggers safe for cross table integrity constraints?
 

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  1. POAre database triggers safe for cross table integrity constraints?
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    <p>I suggested using a trigger to check a cross table integrity constraint answering <a href="https://stackoverflow.com/a/16878683/2032064">this question</a>. It has been suggested in the comments that it may cause problems:</p> <blockquote> <p>Triggers to do cross row checks rarely work on most databases ... because they cannot read uncommitted rows from other transactions</p> </blockquote> <p>I haven't found any source supporting the claim, though. <a href="http://docs.oracle.com/cd/B28359_01/appdev.111/b28843/tdddg_triggers.htm#autoId3" rel="nofollow noreferrer">Official</a> <a href="http://dev.mysql.com/doc/mysql-reslimits-excerpt/5.1/en/stored-program-restrictions.html#stored-routines-function-restrictions" rel="nofollow noreferrer">documentation</a> doesn't mention anything. Others issues I found are covered <a href="https://stackoverflow.com/q/460316/2032064">here on SO</a> - it mostly criticizes potential hidden complexity as triggers are not visible on first sight. Bet even the <a href="https://stackoverflow.com/a/460343/2032064">highest rated answer</a> admits their usage for integrity issues.</p> <p>So my question is: <strong>Are database triggers safe for cross table integrity constraints?</strong> Specially, would the solution below work?</p> <hr> <p>To summarize the original question. We have tables</p> <ul> <li>Player - PlayerID, PlayerName</li> <li>Bet - BetID, BetName</li> <li>plays_in - BetID, PlayerID</li> </ul> <p>The constraint it that the combination of BetName and PlayerID sholud be unique. Definition of the suggested trigger:</p> <pre><code>CREATE TRIGGER check_bet_name BEFORE INSERT ON plays_in FOR EACH ROW BEGIN DECLARE bet_exists INT DEFAULT 0; DECLARE msg VARCHAR(255); SELECT 1 INTO bet_exists FROM Bet AS b1 WHERE b1.BetID = NEW.BetID AND EXISTS (SELECT * FROM plays_in AS p JOIN Bet AS b2 USING (BetID) WHERE p.PlayerID = NEW.PlayerID AND b2.BetName = b1.BetName ) LIMIT 1; IF bet_exists THEN SET msg = "Bet name already exists..."; SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = msg; END IF; END// </code></pre>
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    1. COCan you run a simple test to check that statement about being unable to see uncommitted changes (which sounds like it should be true)?
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    2. CO@Thilo oh my, you're right, it really fails
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