sql - 如何在sql查询中找到记录的第n个孩子

Question

我有一个存储类别和子类别的表（自联接）

The table structure is like this:
-CategoryId
-CategoryName
-ParentCategoryId

如何在sql查询中找到一个类别的第n个孩子/孩子

Answer 1

这样的事情怎么样

DECLARE @Categories TABLE(
        CategoryId INT, 
        CategoryName VARCHAR(20),
        ParentCategoryId INT
)

INSERT INTO @Categories SELECT 1, '1',NULL
INSERT INTO @Categories SELECT 2, '2',NULL

INSERT INTO @Categories SELECT 3, '1.3',1
INSERT INTO @Categories SELECT 4, '1.4',1

INSERT INTO @Categories SELECT 5, '1.3.5',3
INSERT INTO @Categories SELECT 6, '1.3.6',3
INSERT INTO @Categories SELECT 7, '1.3.6.7',6

INSERT INTO @Categories SELECT 8, '1.4.8',4

DECLARE @CatID INT,
        @NthLevel INT
SELECT  @CatID = 1,
        @NthLevel = 2

;WITH Vals AS (
        SELECT  *,
                1 AS CatLevel
        FROM    @Categories c 
        WHERE   CategoryId IS NULL
        UNION ALL
        SELECT  c.*,
                CatLevel + 1 AS CatLevel
        FROM    Vals v INNER JOIN
                @Categories c   ON  c.ParentCategoryId =  v.CategoryID
)
SELECT  *
FROM    Vals
WHERE   CatLevel = @NthLevel

这将递归地构建树结构，并将其限制在您正在寻找的树级别。

Answer 2

SELECT ParentCategoryId,
  , CategoryId AS [n-th child]
FROM (
  SELECT CategoryId
    , ParentCategoryId
    , ROW_NUMBER() OVER (PARTITION BY ParentCategoryId ORDER BY CategoryId) as ChildNumber
  FROM Categories
) p
WHERE ChildNumber = %N

Answer 3

要查找category 的N第 th 个子项，按 CategoryId 排序M：

select  *
from    (
        select  row_number() over (order by c.CategoryId) as rn
        ,       c.*
        from    Categories p
        join    Categories c
        on      p.CategoryId = c.ParentCategoryId
        where   p.CategoryId = M
        )
where   rn = N