UNION和子查询（MySQL）

Question

我什至不确定标题是否正确，但这是简化的情况。

我有一张带有邻接表的表：

comments
- id (int)
- parent_id (int)
- depth_level (int)
- ...

我想做的是用order by和limit查询深度级别0，对于返回的每一行，我想要一个与返回相同表和order by and limit但深度深度不同的查询的联合，我想要那个子查询只返回与父级深度相关的行...等等。 如果有帮助，我可以对深度级别进行限制。 我有点无所适从，像这样：

select * from ( select * from comments where depth = 0 order by id asc LIMIT 10 ) D0
union all 
select * from ( select * from comments where depth = 1 order by id asc LIMIT 10 ) D1

我得到联合行，但是如您所见，我希望D1仅包含具有parent_id和D0 id的行...，我希望将其用于多个级别。 也许这是错误的方法。 我知道这是一厢情愿的想法，但是如果行数超过了所提供的限制，那么我可以以某种方式获得每一行将是很棒的。

一个例子：

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   4           2       Title 8
9   4           2       Title 9
10  4           2       Title 10
11  4           2       Title 11

pseudo:
select * from table where depth = 0 order by id asc limit 1
union 
select * from table where depth = 1 and parent_id from firstQuery.id order by id asc limit 2
union
select * from table where depth = 2 and parent_id from secondQuery.id order by id asc limit 3

result:

id  parent_id   depth   title
1   0           0       Title 1
3   1           1       Title 3
4   1           1       Title 4
8   4           2       Title 8
9   4           2       Title 9
10  4           2       Title 10

编辑2：

扩展peterm的答案。

(
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

但我要限制的是父级PER的深度级别，而不是总深度级别的限制。 像这样（在此示例中，每深度1为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
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

Answer 1

丑陋但是你可以做

(
  SELECT *
   FROM comments
  WHERE depth = 0
  ORDER BY id
  LIMIT 1
)
UNION ALL
(
  SELECT c.*
   FROM comments c JOIN 
  (
    SELECT id
     FROM comments
    WHERE depth = 0
    ORDER BY id
    LIMIT 1
  ) p ON c.parent_id = p.id
   LIMIT 2
)
UNION ALL
(
  SELECT c.*
   FROM comments c JOIN 
  (
    SELECT c.*
     FROM comments c JOIN 
    (
      SELECT id
       FROM comments
      WHERE depth = 0
      ORDER BY id
      LIMIT 1
    ) q ON c.parent_id = q.id
     LIMIT 2
  ) p ON c.parent_id = p.id
   LIMIT 3
)
-- ORDER BY id

输出：

| ID | PARENT_ID | DEPTH |    TITLE |
|----|-----------|-------|----------|
|  1 |         0 |     0 |  Title 1 |
|  3 |         1 |     1 |  Title 3 |
|  4 |         1 |     1 |  Title 4 |
|  8 |         4 |     2 |  Title 8 |
|  9 |         4 |     2 |  Title 9 |
| 10 |         4 |     2 | Title 10 |

这是SQLFiddle演示