Investigate possible support for GROUPING SETS

[AnyhowStep]

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https://www.sqlservertutorial.net/sql-server-basics/sql-server-grouping-sets/

It can be emulated with UNION ALL, since MySQL 5.7 and SQLite 3.30 do not support it. Special care should be taken regarding the GROUPING() function, if choosing to emulate.

[AnyhowStep]

When using grouping sets, columns not in the pairwise intersection of the grouping sets are nullable.

For example, given GROUPING SETS ((id, a), (id, b)), a and b are nullable.

[AnyhowStep]

Related, https://github.com/AnyhowStep/tsql/issues/154