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

["David G. Johnston" <david.g.johnston@xxxxxxxxx>]

On Wednesday, April 1, 2020, David Gauthier <davegauthierpg@xxxxxxxxx> 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 !

You are correct this model seems undesirable but given you have it why doesn’t the following work for you?

Select students where class = ‘math’

Union all

Select all_classes where class = ‘math’

The generally better query is:

Select from students ... group by rollup

https://www.postgresql.org/docs/9.6/queries-table-expressions.html#QUERIES-GROUPING-SETS

David J.