Re: using a common key value on both sides of a union ?

On Wed, 2020-04-01 at 19:39 -0400, David Gauthier wrote:
> psql (9.6.7, server 11.3) on linux
>
> I want to do something like this
> (intentionally bad sql but will illustrate the need)
>
> select s.name,s.grade from students s where s.class='math'
> union
> select 'whole class', class_grade from all_classes where class=s.class
>
> Of course it's that "where class=s.class" that's a foul.
>
> In English, I want a list of each math student and their grade and then append one more record for the entire class, a record that comes from a different table but narrowed down to that one class.
>
> I don't care if union isn't the right approach. Anything that works is welcome !

SELECT s.name, s.grade
FROM students s
WHERE s.class = 'math'
UNION
SELECT 'whole class', a.class_grade
FROM all_classes a
WHERE EXISTS (SELECT 1 FROM students s1
WHERE a.class = s1.class
AND s1.class = 'math');

I deliberately ignored that the condition could be simplified substantially.

Yours,
Laurenz Albe
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