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Grouping and Sorting

It is important to understand that GROUP BY and ORDER BY are very different, even though they often accomplish the same thing. Table 10.1 summarizes the differences between them.

Table 10.1. ORDER BY Versus GROUP BY

ORDER BY	GROUP BY
Sorts generated output	Groups rows. The output might not be in group order, however.
Any columns (even columns not selected) or may be used	Only selected columns or expressions or may be used, and every selected column expression must be used.
Never required	Required if using columns (or expressions) with aggregate functions.

The first difference listed in Table 10.1 is extremely important. More often than not, you will find that data grouped using GROUP BY will indeed be output in group order. But ...