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According to tips from MySQL performance wiki:
Don’t use DISTINCT when you have or could use GROUP BY.
Can somebody post example of queries where GROUP BY can be used instead of DISTINCT?
If you know that two columns from your result are always directly related then it’s slower to do this:
than this:
because in the second case it only has to compare the id, but in the first case it has to compare both fields. This is a MySQL specific trick. It won’t work with other databases.