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Home -> Community -> Usenet -> comp.databases.theory -> Re: Extending my question. Was: The relational model and relational algebra - why did SQL become the industry standard?
"Jan Hidders" <jan.hidders_at_REMOVE.THIS.ua.ac.be> wrote in message
news:3e6e4547.0_at_news.ruca.ua.ac.be...
> Such as combining two iterations over the same bag into one. It's pretty
> easy to see that
>
> SELECT f(x)
> FROM x IN
> SELECT g(y)
> FROM y IN Y
>
> is the same as
>
> SELECT f(g(y))
> FROM y in Y
>
> which is a simple straightforward rule in bag algebra.
Let XY be
x y
- -
1 1
1 2
2 3
Then,
select x from XY
where x in (select y from XY)
returns 1 and 2
while
select y from XY
returns
1,1, and 2
Or my SQL interpretation of your formulas is wrong? Received on Tue Mar 11 2003 - 15:16:42 CST