[DOCS] sql.sgml

[Michael Glaesemann]

Looking through the documentation source, I came across sql.sgml, a 
section that covers relational theory and SQL. It's no longer compiled 
with the docs, afaics, and hasn't been since 7.1.

We often get questions regarding this kind of basic information in 
#postgresql on irc.freenode.net, and people have commented that such a 
reference would be useful. I'd like to be able to point people to such 
a reference in the docs (we currently point then to books or other 
websites).

Perhaps we should include it in the documentation again? Probably needs 
a little polishing, but that can be done. Thoughts?

Michael Glaesemann
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