How can I get the set difference of two result sets?

Say I have a result set (just one column in each):

result1:

'a'

'b'

'c'

result2:

'b'

'c'

I want to minus what is in result1 by result2: result1 - result2 such that it equals:

difference of result1 - result2:

'a'

解決方案

To perform result1 - result2, you can join result1 with result2, and only output items that exist in result1. For example:

SELECT DISTINCT result1.column

FROM result1 LEFT JOIN result2 ON result1.column = result2.column

WHERE result2.column IS NULL

Note that is not a set difference, and won't output items in result2 that don't exist in result1. It's set subtraction.

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