9.19.6.Grouping Sets

The grouping sets variant of group by allows specifying exactly which combinations of groupings are needed. CUBE and ROLLUP are shorthands that expand into a grouping sets specification. This is useful for applications like faceted search where there are separate group by's on properties of interest.

The grouping function behaves identically to CUBE and ROLLUP .

The syntax is:

GROUP BY grouping sets (<grouping set>[,...])

The grouping set is a list of grouping columns in parentheses or () to denote an aggregate without grouping.

For example:

  SELECT key_table, key_is_main, COUNT (*)
GROUP BY grouping sets ((key_table), (key_is_main), ());

This shows for each table the number of indices, then the number of primary key and non-primary key indices across all tables and finally the total number of indices in the schema. A CUBE would produce these same groupings but it would further add the key_table , key_is_main grouping.

As an extension to SQL 99 OLAP extensions, the Virtuoso grouping sets specification also supports an ORDER BY with optional top. In the place of a grouping set one can write:

ORDER BY [top k] (<column> [,...])

In this way a single query can produce a set of ordered result rows and different grouped aggregates on the columns in the result set.