Oracle FAQ | Your Portal to the Oracle Knowledge Grid |
Home -> Community -> Usenet -> comp.databases.theory -> Re: Extending my question. Was: The relational model and relational algebra - why did SQL become the industry standard?
Mikito Harakiri wrote:
>"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?
No, I messed the notation a bit up because I thought it would be more clear. What I meant was:
>> SELECT f(x)
>> FROM
>> ( SELECT g(y)
>> FROM y IN Y ) AS x
and
>> SELECT f(g(y))
>> FROM Y AS y