Question

Given theses tables:

create temporary table a(pid integer, cid integer);
create temporary table b(id integer, code varchar);

This works but returns wrong results:

select pid 
from a 
where cid in (select cid from b where code like 'AE%')

I just had a query like that where I use the wrong field, and it surprised me that that query even works. Doesn’t a query like this just return all the rows from table a ?

Do you have any example of a query written like this that would be useful ?

Maybe I’m missing something.

Answer 1

You regulary need fields from the outer query in the where clause of the inner query:

 select * from a
    where exists ( select 1 from b where id = cid );

or

 select * from a
    where 'HELLO' in (select code from b where id = cid );

We can also construct examples where the outer fields are (kind of) useful in the select clause:

 select * from a
    where 1 = any (select id-cid from b where code like 'HE%');

Therefore, access to the fields of the outer query is absolutely necessary.