That is: a sequence of operations A -> B -> C is represented as SELECT C FROM SELECT B FROM SELECT A. However, the SQL 99 standard introduced t

Using WITH For Neater SQL – Win Vector LLC

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2021-06-21 04:00:08

That is: a sequence of operations A -> B -> C is represented as SELECT C FROM SELECT B FROM SELECT A. However, the SQL 99 standard introduced the WITH statement and common table expressions (ref). WITH statements allow forward composition.

It is interesting to note when WITH or common table expressions became widely available. The Wikipedia has the versions (and hence dates) (ref) showing when common table expressions are supported in the following databases.

Some of the cost of implementing common table expressions, is they are where databases allow recursive or fixed-point semantic extensions. From the database point of view these are major semantic changes, not mere notational conveniences.