Re: How do you write this query?

On Thursday 31 Oct 2002 6:21 pm, Wei Weng wrote:
>  data | data1 | data2
> ------+-------+-------
>     1 | foo   | bar
>     2 | greg  | bar
>     3 | pooh  | bar
>     4 | dah   | peng
>
> I need a query that returns me the "data1" that satisfies the logic of
> the following pseudo code:
>
> 1: select data2 into @out from test where data1 = 'pooh'
> 2: select data1 from test where data2 = @out and data = 3

The most literal would be something like:

SELECT t1.data1 FROM test t1
WHERE t1.data=3 AND t1.data2 IN   (SELECT t2.data2    FROM test t2    WHERE t2.data1='pooh')

You can probably get away without the t1/t2 stuff but that should make things
clear.

Since Postgresql isn't very good at optimising IN, you might want to rewrite
it as an EXISTS query instead - see the manuals and mailing list archives for
details.

HTH
--  Richard Huxton