MySQL UNION

[geraldus]

Consider following question on StackOverflow: http://stackoverflow.com/questions/4230780/mysql-query-with-multiple-limits
I have similar task, but I can't find union function in Esqueleto. Can I introduce similar single query using Esqueleto or should I make several queries?
Thank you.

[ocharles]

I am also surprised at the lack of UNION - any idea roughly how hard this would be?

[meteficha]

I don't foresee any problems, @ocharles, it shouldn't be too hard. Most work is probably going to be on the test suite :).

[meteficha]

I took a stab at this issue and it's actually a lot more complicated than I anticipated :). I've tried adding:

union :: [query a] -> query a -- resp. unionAll

My first approach was morphing SideData into a monster GADT that basically amounted to:

    RealSideData {...as beore...}
  | UnionSideData [(a, RealSideData)]

This approach failed because of the SqlSelect typeclass. The one available at toRawSql can't be used because it's not available when UnionSideData is created. And we don't have a SqlSelect constraint on union.

I was planning on adding an an associated type family to the Esqueleto type class in order to change the type of union to UnionConstraint a => [query a] -> query a, which I think would solve the issue.

Then I realize that the whole SideData contraption could be removed in favor of adding a new constructor to SqlExpr and calling toRawSql directly on union's implementation. At this point I've put this endeavor to a pause.