有人可以帮我解决这个问题吗？

Question

该表中的每一行都包含一个名为 ParentFamilyId 的列，它告诉您任何家庭属于哪个父家庭（所有类别家庭 - 编号 25 - 没有父级，因此位于层次结构的顶部）。

创建一个使用外连接链接 3 个表的查询，如下所示：

Table       Alias
tblFamily   Family
tblFamily   ParentFamily
tblFamily   TopFamily

将计算列添加到您的查询中，以便它显示所有 25 个系列：

如果您按 FamilyName 顺序显示家庭，您应该会看到类似这样的内容。

这是我到目前为止：

SELECT Family.FamilyName FamilyName,
       CASE WHEN Family.FamilyName = 'All categories' THEN Family.FamilyName 
            ELSE CONCAT(topfamily.FamilyName, '>', Parentfamily.FamilyName, '>', Family.FamilyName) 
       END AS 'Family path'
FROM tblFamily Family
LEFT JOIN tblFamily ParentFamily ON Family.ParentFamilyId = ParentFamily.FamilyID
LEFT JOIN tblFamily TopFamily ON TopFamily.FamilyName = 'All categories'
order by Family.FamilyName

我有正确的结果，但这是最好的方法吗？ 很长一段时间以来，我都与开发脱节，所以练习基础知识。

Answer 1

创建一个使用外连接链接 3 个表的查询，如下所示：
Table Alias
tblFamily Family
tblFamily ParentFamily
tblFamily TopFamily

我会说除非你的老师打算用这个例子作为你应该使用递归 ctes 的原因，否则这是一个糟糕的家庭作业。 话虽如此，您的查询实际上与正确答案相去甚远：

SELECT COALESCE(Family.FamilyName, Parentfamily.FamilyName, topfamily.FamilyName) As 'Family Name',
       topfamily.FamilyName + 
       ISNULL('>' + Parentfamily.FamilyName, '') + 
       ISNULL('>' + Family.FamilyName, '') As 'Family path'
FROM tblFamily As TopFamily 
LEFT JOIN tblFamily As ParentFamily ON ParentFamily.ParentFamilyId = TopFamily.FamilyID
LEFT JOIN tblFamily As Family ON Family.ParentFamilyId = ParentFamily.FamilyID
WHERE TopFamily.ParentFamily.ParentFamilyId IS NULL -- So of someone changes the `All categories` text your query will not have to change
order by Family.FamilyName

笔记：

• 您的第一列Family.FamilyName FamilyName,是错误的，因为Family可能并不总是包含行。 此外， 始终对别名使用as关键字。
  我的第一列使用COALESCE来获取三个别名的第一个非空值。

• 您的第二列假定它是 1 级或 3 级，但图片显示并非如此。 在我的第二列中，我利用了在null和 varchar 值之间使用+连接运算符为您提供 null 的Family.FamilyName - 因此，如果Family.FamilyName为 null，结果将是topfamily.FamilyName > Parentfamily.FamilyName .

• 外连接中表的顺序很关键。 由于始终会选择TopFamily别名，因此它必须是 from 子句中的第一个表，然后是ParentFamily别名，最后是Family别名。

• 表之间的连接总是在右 table.id = left table.parentId

• where 子句现在包含一个条件，该条件对于顶级系列始终为真，即使您有多个或名称更改。

Answer 2

就像@ZLK 建议的那样，您可能想尝试递归 CTE。 这是一个例子。

;WITH
tblFamily
AS
(
    SELECT tbl.* FROM (VALUES
      ( 1, 0, 'All categories')
    , ( 1, 2, 'Something1')
    , ( 1, 3, 'Something2')
    , ( 1, 4, 'Something3')
    , ( 1, 5, 'Something4')
    , ( 1, 6, 'Something5')
    , ( 2, 10, 'Something10')
    , ( 3, 11, 'Something11')
    , ( 4, 12, 'Something12')
    , ( 5, 13, 'Something13')
    , ( 6, 14, 'Something14')
    ) tbl ([ParentFamilyId], [FamilyID], [FamilyName]) 
)
,
tblFamily_relate ([ParentFamilyId], [FamilyID], [FamilyName], [FamilyTree], [FamilyLevel])
AS
(
    SELECT 
          Family.[ParentFamilyId]
        , Family.[FamilyID]
        , Family.[FamilyName]
        , [FamilyTree] = cast(Family.[FamilyName] as varchar(max))
        , [FamilyLevel] = 0
    FROM 
        tblFamily Family
    WHERE 
        Family.[FamilyID] IN(SELECT [ParentFamilyId] FROM tblFamily)
    UNION ALL
    SELECT 
          Family.[ParentFamilyId]
        , Family.[FamilyID]
        , Family.[FamilyName]
        , [FamilyTree] = cast(ParentFamily.[FamilyName] as varchar(max)) + cast(Family.[FamilyName] as varchar(max))  
        , [FamilyLevel] + 1
    FROM 
        tblFamily Family
        INNER JOIN tblFamily_relate ParentFamily  ON Family.ParentFamilyId = ParentFamily .FamilyID
    WHERE 
        Family.[FamilyID] NOT IN(SELECT [ParentFamilyId] FROM tblFamily)
)
SELECT * FROM tblFamily_relate