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<IN vs. JOIN vs. EXISTS ⁄ >




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Before we begin I should note that the queries provided by the author are not similar in fact and could produce different resultsets.

If the table used in the subquery returns a value twice, a JOIN will also return the matching rows twice, while an IN condition will return them only once.



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http://explainextended.com/2009/06/16/in-vs-join-vs-exists/




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