MySQL select all subquery rows with minimum value - mysql

Question

I'd like to select all rows of a subquery with the minimum value in a given field. Here's some toy examples of the techniques I've tried so far:
-- 1.
select
id, min(foo)
from
(select 1 AS id, 2 AS foo
union select 2 AS id, 2 AS foo
union select 3 AS id, 3 AS foo) a;
-- 2.
select
min(foo)
from
(
select 1 AS id, 2 AS foo, 0 AS const
union select 2 AS id, 2 AS foo, 0 AS const
union select 3 AS id, 3 AS foo, 0 AS const) a
group by const;
-- 3.
select
id
from
(select 1 AS id, 2 AS foo
union select 2 AS id, 2 AS foo
union select 3 AS id, 3 AS foo) a
where id = (select id from a where min(foo) = foo);
-- 4.
select
id
from
(select 1 AS id, 2 AS foo
union select 2 AS id, 2 AS foo
union select 3 AS id, 3 AS foo) a
where foo = (select min(foo));
-- 5.
select r.*
from
(
select min(foo) t
from
(select 1 AS id, 2 AS foo
union select 2 AS id, 2 AS foo
union select 3 AS id, 3 AS foo) a
) m
INNER JOIN a ON m.t = r.foo;
The actual query I'm working on is similar to the examples, in that it's made up of several smaller queries UNIONed together. The overall goal here is to lookup a row in a central table based on the fields of an association table k to which it is joined, where k is highest priority table. The result is a sort of tree view of rows from similar (but different tables).
I've mentioned this so in case someone can see that I'm going about this in a roundabout way they can shed some light on the bigger picture. But for now my angle is to select by a taking a minimum value on a field in the subquery.

Answer 1

Use order by and limit:
select t.*
from t
order by foo
limit 1;
Note: this only returns one row with the minimum, even if there are duplicates. The t is your subquery or table.
If you want all of them, then you need to include the table definition twice:
select t.*
from t
where t.foo = (select min(t2.foo) from t t2);