I was wondering if there is any difference between order of grouping in GROUP BY a, b and GROUP BY b, a (I know the final result is the same). If so, would it affect the query's speed?
GROUP BY a, b VS GROUP BY b, a
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2 Answers
A group by clause just defines the unique combination of field(s) which would be considered a group. There is no meaning to the order these fields are stated.
It does matter if you have multiple-column indexes. You should define the GROUP BY columns in the order of the index.
So, if you have an index for (a,b) then you should use GROUP BY a, b and MySQL is able to take full advantage of the index.
See example
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