MySQL查询: LIMITing a JOIN

Question

假设我有两个表，我想加入。类别：

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

和项目：

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

我想要一个返回类别和第一个(并且只返回第一个)项名称的查询：

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

有没有一种方法可以得到随机的结果，比如：

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

谢谢!

Answer 1

只是做了个快速测试。这似乎起作用了：

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)

我认为这将满足您的第一个问题。我不确定第二个--我认为它需要一个内部查询，使用order by random()或类似的东西！

Answer 2

Mysql允许您拥有不包含在分组或聚合中的列，在这种情况下，它们具有随机值：

    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

或者，对于最小值，

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 3

Mysql确实允许包含非聚合列，并且不能保证确定性，但在我的经验中，我几乎总是得到第一个值。

所以通常(但不能保证)这会给你第一个

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

如果你想得到保证，你需要做一些事情，比如

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)

如果您想要随机答案，则必须稍微更改子查询

 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)