if-not-exists is a common SQL condition used to check if an entity exists.

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MySQL: NOT EXISTS vs LEFT OUTER JOIN…IS NULL

Is NOT EXISTS or LEFT OUTER JOIN...IS NULL more efficient for finding records in one table that aren't in another? Specifically, I'm trying to figure out which of the two queries below is better: ...
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Adding a constant IF not in an other table

So I have this query. SELECT * FROM `Comment` WHERE `Target` = '".$_GET['pp']."' AND `Id` NOT IN (SELECT `Target` FROM `Delete` WHERE `Type` = 'comment') ORDER ...