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<Exists Vs. Count(*) - The battle never ends... ⁄ >




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I am still amazed at how many of the database applications written today still disregard some basic rules of thumb when it comes to accessing the data. One in particular is the use of COUNT(*) to check to see if there are any rows that match some criteria. The technique of using EXISTS over COUNT(*) has been widely publicized and is in pretty much every best practices document I have come across. So why are database developers still using COUNT(*) instead of EXISTS all over the place? If it is because people just don't believe or recognize the benefits of it? Or is the concept of EXISTS difficult for new programmers to grasp? I am not really sure since I have heard both views. So to cover both bases I will show a little demo of why EXISTS is almost always a better way to code when you simply need to see if there is at least 1 row that matches some condition in the WHERE clause. Of course if you really need to know exactly how many match that condition then COUNT(*) is appropriate so hopefully this won't confuse anyone in that regard.



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http://sqlblog.com/blogs/andrew_kelly/archive/2007/12/15/exists-vs-cou...




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