MySQL group-by very slow

Question

I have the folowwing SQL query

SELECT CustomerID FROM sales WHERE `Date` <= \'2012-01-01\' GROUP BY CustomerID

The query is executed over 11

Answer 1

Wouldn't this one be a lot faster and achieve the same?

SELECT DISTINCT CustomerID FROM sales WHERE `Date` <= '2012-01-01'

Make sure to place an index on Date, of course. I'm not entirely sure but indexing CustomerID might also help.