Continuing with the mini-series on query operators, I want to have a look at NOT EXISTS and NOT IN.Just one note before diving into that. The examples I’m using are fairly simplistic and that’s intentional. I’m trying to find what, if any, are the performance differences in a benchmark-style setup. I’ll have some comments on more complex examples in a later post. The most important thing to note about NOT EXISTS and NOT IN is that, unlike EXISTS and IN, they are not equivalent in all cases. Specifically, when NULLs are involved they will return different results. To be totally specific, when the subquery returns even one null, NOT IN will not match any rows. The reason for this can be found by looking at the details of what the NOT IN operation actually means.Read more: SQL in the Wild