MySQL Query: LIMITing a JOIN

Question

Say I have two tables I want to join. Categories:

id   name
----------
1    Cars
2    Games
3    Pencils

And items:

id   categoryid   itemname
---------------------------
1    1            Ford
2    1            BMW
3    1            VW
4    2            Tetris
5    2            Pong
6    3            Foobar Pencil Factory

I want a query that returns the category and the first (and only the first) itemname:

category.id category.name item.id item.itemname
-------------------------------------------------
1           Cars          1       Ford
2           Games         4       Tetris
3           Pencils       6       Foobar Pencil Factory

And is there a way I could get random results like:

category.id category.name item.id item.itemname
-------------------------------------------------
1           Cars          3       VW
2           Games         5       Pong
3           Pencils       6       Foobar Pencil Factory

Thanks!

Answer 1

Mysql lets you to have columns not included in grouping or aggregate, in which case they've got random values:

    select category.id, category.name, itemid, itemname
    inner join 
      (select item.categoryid, item.id as itemid, item.name as itemname
       from item group by categoryid)
    on category.id = categoryid

Or, for minimums,

select category.id, category.name, itemid, itemname
inner join 
  (select item.categoryid, min(item.id) as itemid, item.name as itemname
  from items
  group by item.categoryid)
on category.id = categoryid

Answer 2

Just done a quick test. This seems to work:

mysql> select * from categories c, items i
    -> where i.categoryid = c.id
    -> group by c.id;
+------+---------+------+------------+----------------+
| id   | name    | id   | categoryid | name           |
+------+---------+------+------------+----------------+
|    1 | Cars    |    1 |          1 | Ford           |
|    2 | Games   |    4 |          2 | Tetris         |
|    3 | Pencils |    6 |          3 | Pencil Factory |
+------+---------+------+------------+----------------+
3 rows in set (0.00 sec)

I think this would fulfil your first question. Not sure about the second one - I think that needs an inner query with order by random() or something like that!

Answer 3

Mysql does let include non aggregate columns and there is no guarantee of determinism, but in my experience I nearly always get the first values.

So usually (but not guaranteed) this will give you the first

select * 
from categories c, items i
where i.categoryid = c.id
group by c.id;

If you want guaranteed you will need to do something like

select categories.id, categories.name, items.id, items.name
from categories inner join
  items on items.categoryid = categories.id and 
     items.id = (select min(items2.id) from items as items2 where items2.categoryid = category.id)

If you want random answers you will have to change the subquery a little bit

 select categories.id, categories.name, items.id, items.name
    from categories inner join
      items on items.categoryid = categories.id and 
         items.id = (select items2.id from items as items2 where items2.categoryid = category.id order by rand() limit 1)