[jira] [Created] (CALCITE-2224) WITHIN GROUP clause for aggregate functions

["Julian Hyde (JIRA)" <ji...@apache.org>]

Julian Hyde created CALCITE-2224:
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             Summary: WITHIN GROUP clause for aggregate functions
                 Key: CALCITE-2224
                 URL: https://issues.apache.org/jira/browse/CALCITE-2224
             Project: Calcite
          Issue Type: Bug
            Reporter: Julian Hyde
            Assignee: Julian Hyde


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|https://docs.oracle.com/cloud/latest/db112/SQLRF/functions089.htm#SQLRF30030] that concatenates strings. E.g.
{code:java}
SELECT LISTAGG(last_name, '; ')
         WITHIN GROUP (ORDER BY hire_date, last_name)
  FROM Emp
GROUP BY deptno{code}
{{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|https://dev.mysql.com/doc/refman/5.7/en/group-by-functions.html#function_group-concat] 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}}.



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