Re: Getting several columns from subselect with LIMIT 1

On Sat, Sep 20, 2008 at 11:36 AM, Pierre Thibaudeau <pierdeux@gmail.com> wrote:
> I have a PERSONS table.
> I also have a PROGENY table, which is a many-to-many association table
> with two foreign keys to the PERSONS table to itself.
> (In this day and age, not only can an individual have any number of
> children, but also a person can have any number of parents!  At least,
> let's assume that's true for the sake of this setup.)
>
> Suppose I wish to construct a view of the persons, along with the name
> of their first-born (if they have one;  NULL otherwise).  The
> following SELECT does just that:
>
> SELECT
>        persons.*,
>        (
>                SELECT child.name
>                FROM progeny JOIN persons child ON child.id = progeny.child
>                WHERE progeny.parent = persons.id
>                ORDER BY child.birthdate ASC
>                LIMIT 1
>        ) AS firstborn_name
>  FROM persons;
>
> Now, this is probably not the most elegant piece of code, but the real
> problem is that
> I cannot see how to extend it to the case where I want not only the
> firstborn's name but also the firstborn's ID
> (short of repeating the entire subselect a second time).  At the
> moment, with this current syntax, my subSELECT statement would not be
> allowed to return more than a single column.



SELECT (person).*, (progeny).* from
(
   select persons as person,
       (
               SELECT progeny
               FROM progeny JOIN persons child ON child.id = progeny.child
               WHERE progeny.parent = persons.id
               ORDER BY child.birthdate ASC
               LIMIT 1
       ) AS firstborn
  FROM persons;
) q;