Sort MySql query by field in exists sub query - mysql

Question

I am looking for a way to sort a query by a field that exists within a where exists query.
An attempt to use a field from the exists subquery I get the 'Unknown column' error.
Example query;
select
*
from
`table_a`
where
`a_field` = 'foo'
and exists (
select
*
from
`table_b`
where
`table_a`.`an_id` = `table_b`.`another_id` and `table_b`.`another_field` = 'bar'
)
order by
`table_a`.`created_at` asc,
`table_b`.`another_id` desc;
Is the only solution to use an inner join query like;
select
`t1`.*
from
`table_a` as `t1`
inner join `table_2` as `t2` on `t2`.`another_id` = `t1`.`an_id`
where
`t1`.`a_field` = 'foo'
and `t2`.`another_field` = 'bar'
order by
`t1`.`created_at` asc,
`t2`.`another_id` desc;

Answer 1

Your example query is ordering by another_id, which is used in the correlation clause. So, you can just do:
select a.*
from table_a a
where a.a_field = 'foo' and
exists (select 1
from table_b b
where a.an_id = b.another_id and
b.another_field = 'bar'
)
order by a.created_at asc,
a.an_id desc;
Assuming you actually want a different column, you can use a JOIN. The issue is that more than one row may match. So you need to remove the duplicates in the subquery:
select a.*
from table_a a join
(select b.another_id, max(b.another_col) as another_col
from table_b b
where another_field = 'bar'
group by b.another_id
) b
on a.an_id = b.another_id
where a.a_field = 'foo'
order by a.created_at asc, b.another_id desc;
You can only use your form of the JOIN if you know that at most one row will match.