"Andy Turk" <andy_turk(at)hotmail(dot)com> writes:
> I was reading Graeme Birchall's SQL Cookbook at
> and came across an *amazing* technique called recursive SQL.
Interesting, but I think Birchall has confused some very peculiar
(and incorrect) implementation-specific behavior of DB2 with SQL.
This is not SQL.
Leaving aside a minor quibble about whether the WITH syntax he shows
is valid (it's surely not SQL92, although it might be SQL3 if SQL3 ever
becomes a standard), the really fundamental problem is that you cannot
have a SELECT query that inspects its own output. He claims that in
SELECT foo UNION SELECT bar, the "bar" select will somehow see the
output of the "foo" select --- and not only that, but will be
recursively invoked to see its *own* outputs. I do not believe that
any such interpretation can be extracted from the SQL standard.
If SQL worked that way, then simple commands like
UPDATE foo SET x = 42 WHERE y = 44
would be infinite loops, because they'd see the new tuples produced
by their own action and try to update those, leading to more new
tuples, etc etc.
He's built a large intellectual edifice on a DB2 bug.
regards, tom lane
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