mysql - 在右表上加入限制

Question

假设我有两个要加入的表。分类：

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

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最后一个最大 N（例如：2）项目名称的查询：

category.id category.name item.id item.itemname
-------------------------------------------------
1           Cars          2       BMW
1           Cars          3       VW
2           Games         4       Tetris
2           Games         5       Pong
3           Pencils       6       Foobar Pencil Factory
4           Books         NULL    NULL

我想写一个如下查询：

Select * From categories c
 Left Join (select * from items order by id desc) i
 On c.id=i=categoryid
  AND LIMIT 2 #comment: N=2 this line not supported
 Where i.categoryid = c.id
 Group By c.id

谢谢！

Answer 1

我并不是说它有效，但它应该有效：

SELECT c.*, i.id, i.itemname
FROM categories c
LEFT JOIN
  (SELECT i.*
   FROM items i
   LEFT JOIN items i2 ON i.categoryid = i2.categoryid AND i2.id > i.id
   GROUP BY i.id
   HAVING COUNT(*) < 2         # this 2 = N
  ) i ON c.id = i.categoryid

Answer 2

检查http://sqlfiddle.com/#!2/9a132/1

SELECT *
FROM Categories c 
LEFT JOIN -- Items i 
(
    SELECT * FROM Items WHERE LOCATE(id,
    (
        SELECT GROUP_CONCAT(it.itemids) AS its
        FROM (
                SELECT (SUBSTRING_INDEX(GROUP_CONCAT(CONVERT(id, CHAR(8))
                                                     ORDER BY id DESC), ',', 2)) AS itemids
                FROM Items
                GROUP BY categoryid
            ) it
    )) <> 0
) i
ON i.categoryid = c.id;

其中 N=2 是：SUBSTRING_INDEX(GROUP_CONCAT(CONVERT(id, CHAR(8)) ORDER BY id DESC), ',', 2)

因为 GROUP_CONCAT 默认是 sort ASC ；以上将变为 SUBSTRING_INDEX(GROUP_CONCAT(CONVERT(id, CHAR(8)) ), ',', -2)。但是 GROUP_CONCAT 结果被截断为 max len 1024 (group_concat_max_len)。