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mysql “NOT IN” not working as expected

Last night, I was trying to extract all users that were not listed in one of several other tables. Thinking that I could simply use


and receiving a result, I was on my way.

However, after analyzing the data a little bit I was surprised to see records in the set that should not have been there. “NOT IN” was not working as I expected it to.

A quick search also yielded similar experiences from other users on stackoverflow. I changed the query to use LEFT JOIN as follows:

SELECT * FROM users LEFT JOIN othertable on = WHERE IS NULL

This query yields the expected data and according to the stackoverflow discussion even performs a little better. It is easy extendable to more than two tables as well.

Time to go back and check similar code I’ve used in the past!


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