Affects Version/s: None
Fix Version/s: 1.18.0
The WITHIN GROUP clause lets aggregate functions operate on a sorted list of rows, rather than the usual unsorted collection. Order only matters for a few aggregate functions, but we should allow it for all.
Other analytic functions where WITHIN GROUP would have an effect: RANK, PERCENT_RANK, FIRST_VALUE, LAST_VALUE, PERCENTILE_CONT, PERCENTILE_DISC.
LISTAGG(value [, separator]) is an Oracle function that concatenates strings. E.g.
STRING_AGG(value [, separator]) is a [Microsoft SQL Server function|] that does something similar.
GROUP_CONCAT(value [, separator] [ORDER BY expr [, expr]...) is the MySQL equivalent to LISTAGG. Note the optional ORDER BY clause within the parentheses.
COLLECT(value) is a SQL standard aggregate function that creates multisets. Oracle added a non-standard ORDER BY clause within the parentheses.
In my opinion, WITHIN GROUP should always be optional. LISTAGG without WITHIN GROUP would produce non-deterministic output (which is OK); other aggregate functions such as MIN and SUM would just ignore WITHIN GROUP.