Why is ORDER BY in a FROM subquery ignored?
A query like
SELECT field1, field2 FROM ( SELECT field1, field2 FROM table1 ORDER BY field2 ) alias
returns a result set, not necessarily ordered by field2. It is not a bug.
A "table" (and subquery in the FROM
clause too) is - according to the SQL standard - an unordered set of rows. Rows in a table (or in a subquery in the FROM
clause) do not come in any specific order. That's why the optimizer can ignore the ORDER BY
clause that you have specified. In fact, SQL standard does not even allow the ORDER BY
clause to appear in this subquery (we allow it, because ORDER BY ... LIMIT ...
changes the result, the set of rows, not only their order).
You need to treat the subquery in the FROM
clause, as a set of rows in some unspecified and undefined order, and put the ORDER BY
on the top-level SELECT
.