kefeizhou kefeizhou - 1 year ago 90
SQL Question


If I want to find a set of entries in table A but not in table B, I can use either LEFT OUTER JOIN or NOT EXISTS. I've heard SQL Server is geared towards ANSI and in some case LEFT OUTER JOINs are far more efficient than NOT EXISTS. Will ANSI JOIN perform better in this case? and are join operators more efficient than NOT EXISTS in general on SQL Server?

Answer Source

Joe's link is a good starting point. Quassnoi covers this too.

In general, if your fields are properly indexed, OR if you expect to filter out more records (i.e. have a lots of rows EXIST in the subquery) NOT EXISTS will perform better.

EXISTS and NOT EXISTS both short circuit - as soon as a record matches the criteria it's either included or filtered out and the optimizer moves on to the next record.

LEFT JOIN will join ALL RECORDS regardless of whether they match or not, then filter out all non-matching records. If your tables are large and/or you have multiple JOIN criteria, this can be very very resource intensive.

I normally try to use NOT EXISTS and EXISTS where possible. For SQL Server, IN and NOT IN are semantically equivalent and may be easier to write. These are among the only operators you will find in SQL Server that are guaranteed to short circuit.

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