Question

我之前尝试过提问，但如果没有正确的术语，我很难提出具体的问题，我并不熟悉。所以这是一个例子

以此查询为例：

(
SELECT *
FROM comments
WHERE depth = 0
ORDER BY id DESC
LIMIT 2
)
UNION ALL
(
  SELECT c.*
    FROM comments c JOIN 
    (
      SELECT id
      FROM comments
      WHERE depth = 0
      ORDER BY id DESC
      LIMIT 2
    ) p ON c.parent_id = p.id
    LIMIT 5
)


id  parent_id   depth   title
1   0           0       Title 1
2   0           0       Title 2
3   1           1       Title 3
4   1           1       Title 4
5   1           1       Title 5
6   1           1       Title 6
7   1           1       Title 7

我得到两个深度0行，并且在连接中我也得到了这两个返回查询的5个子元素。我想得到的是获得这两个查询中的每一个的5个子元素，总共10行（深度为1）。例如：

id  parent_id   depth   title
1   0           0       Title 1
2   0           0       Title 2
3   1           1       Title 3
4   1           1       Title 4
5   1           1       Title 5
6   1           1       Title 6
7   1           1       Title 7
8   2           1       Title 8
9   2           1       Title 9
10  2           1       Title 10
11  2           1       Title 11
12  2           1       Title 12

是否可以使用邻接列表以及将所有内容作为union（flat）返回的要求？

编辑：感谢Bill Karwin的回答，我现在就开始工作了。我还在想是否有更短的方式来写这个。我有6（0-5）深度级别，所以我的查询相当长（可能不是最优）。这是三个级别的样子（你可以想象完整的那个）。

-- DEPTH LEVEL 0
(
  SELECT * FROM (
    SELECT *, 1 as _rn, @parent:=0
    FROM comments
    WHERE depth = 0
    ORDER BY id DESC
    LIMIT 2
  ) as D0
)
union all

-- DEPTH LEVEL 1
(
SELECT *
FROM (
  SELECT c.*, @row:=IF(@parent=c.comment_id,@row+1,1) AS _rn, @parent:=c.comment_id
    FROM (SELECT @parent:=null) AS _init 
    STRAIGHT_JOIN comments c 
    INNER JOIN 
    (
      SELECT id
      FROM comments
      WHERE depth = 0
      ORDER BY id DESC
      LIMIT 2
    ) p ON c.comment_id = p.id
  ) AS _ranked
WHERE _ranked._rn <= 5
)
union all

-- DEPTH LEVEL 2
(
SELECT *
FROM (
  SELECT c.*, @row:=IF(@parent=c.comment_id,@row+1,1) AS _rn, @parent:=c.comment_id
    FROM (SELECT @parent:=null) AS _init 
    STRAIGHT_JOIN comments c 
    INNER JOIN 
    (

      (
      SELECT *
      FROM (
        SELECT c.*, @row:=IF(@parent=c.comment_id,@row+1,1) AS _rn, @parent:=c.comment_id
          FROM (SELECT @parent:=null) AS _init 
          STRAIGHT_JOIN comments c 
          INNER JOIN 
          (
            SELECT id
            FROM comments
            WHERE depth = 0
            ORDER BY id DESC
            LIMIT 2
          ) p ON c.comment_id = p.id
        ) AS _ranked
      WHERE _ranked._rn <= 2
      )

    ) p ON c.comment_id = p.id
  ) AS _ranked
WHERE _ranked._rn <= 2
)

Answer 1

您不能使用LIMIT执行此操作，因为在完成所有加入，分组，排序等结果集完成后应用LIMIT。

您正在使用greatest-n-per-group类型查询的变体。在MySQL中执行此操作非常棘手，因为MySQL不支持许多其他SQL数据库支持的ROW_NUMBER（）窗口函数。

这是MySQL的一种解决方法，其中用户定义的变量可以取代分区的行号：

SELECT *
FROM (
  SELECT c.*, @row:=IF(@parent=c.parent_id,@row+1,1) AS _rn, @parent:=c.parent_id
    FROM (SELECT @parent:=null) AS _init 
    STRAIGHT_JOIN comments c 
    INNER JOIN 
    (
      SELECT id
      FROM comments
      WHERE depth = 0
      ORDER BY id DESC
      LIMIT 2
    ) p ON c.parent_id = p.id
  ) AS _ranked
WHERE _ranked._rn <= 5

每个连接的MySQL查询限制

1 个答案: