Procedurally transform subquery into join

Question

Is there a generalized procedure or algorithm for transforming a SQL subquery into a join, or vice versa? That is, is there a set of typographic operations that can be appli

Answer 1

This rates a strong "it depends".

At one level, if you're talking about queries compatible with ANSI SQL 89 or 92*, then I would guess it's a definite maybe. If you have simple (or even not so simple) queries consisting of "basic" select, from, and where clauses, then yes, I would like to think that it is mathematically possible to define processes and procedures to create and "uncreate" subqueries (though how you might determine when to algorithmically form a subquery is beyond me). I think this "rationale" could be applied to outer joins and correlated subqueries.

At another level, I'd say "no way". Most of the time I write a subquery, it's because I can't think of a way to wedge it into the "main" query. Very rarely this involves correlated subqueries, but more often than not in involves what are, I'm pretty darn sure, proprietary extensions to the standards. How could you account for pivots, unpivots, ranking functions, TOP N clauses (which may well be ANSI standards, I'll admit to never having read them cover to cover), FULL or OUTER APPLY, and the like? And that's just parts of SQL Server, I'm sure Oracle, DB2, MYSQL, and most every other player has their own extensions that break the "purist" relational model.

Of course, they say it is impossible to prove a negative. I'd summarize with "can't be done until proven otherwise", leave the proof to the academics and theoreticians, and point out that even then, whatever system you purchase won't support it unless it makes financial sense for the manufacturer to work it in. (Does any system support OUTER UNION yet?)

** A bit of googling failed to produce any references to a third ANSI SQL standard. I know I heard talk about it years ago, did it ever happen?*