帮助与递归SQL

Question

我有一个类似的类别表：

uid | ParentLevel | ParentID | Name 
    ------------------------------------ 
    1 |  0  |  0 | foo 
    2 |  1  |  1 | blat 
    3 |  1  |  1 | baz 
    4 |  2  |  3 | blah 
    5 |  0  |  0 | bar 

我试图得到类似的输出：

1 | foo 
2 | foo | blat 
3 | foo | baz 
4 | foo | baz | blah 
5 | bar 

等。 CTE的都出来了，因为这是SQL 2000年我曾尝试这样的事情，但似乎无法得到它完全正确：

SELECT  c1.uid, c1.Name, c2.Name AS Expr1, c3.Name AS Expr2 
FROM   dbo.Categories AS c1 INNER JOIN 
        dbo.Categories AS c2 ON c1.uid = c2.ParentID INNER JOIN 
        dbo.Categories AS c3 ON c2.uid = c3.ParentID 
WHERE  (c1.ParentLevel = 0) AND (c2.ParentLevel = 1) AND (c3.ParentLevel = 2) 

这只会返回记录如果有第三个层次，这并不总是真正。有没有办法做到这一点？

Answer 1

使用LEFT JOIN代替INNER JOIN和水平从WHERE到JOIN条件

SELECT  c1.uid, c1.Name, c2.Name AS Expr1, c3.Name AS Expr2 
FROM  dbo.Categories AS c1 
LEFT JOIN dbo.Categories AS c2 
     ON c1.uid = c2.ParentID 
     AND c2.ParentLevel = 1 
LEFT JOIN dbo.Categories AS c3 
     ON c2.uid = c3.ParentID 
     AND c3.ParentLevel = 2 
WHERE  c1.ParentLevel = 0 

Answer 2

使用LEFT JOIN

SELECT  c1.uid, c1.Name, c2.Name AS Expr1, c3.Name AS Expr2 
FROM   dbo.Categories AS c1 LEFT JOIN 
        dbo.Categories AS c2 ON c1.uid = c2.ParentID LEFT JOIN 
        dbo.Categories AS c3 ON c2.uid = c3.ParentID 
WHERE  (c1.ParentLevel = 0) AND (c2.ParentLevel = 1) AND (c3.ParentLevel = 2) 

Answer 3

向左移动条款加入将帮助您得到预期的结果。 INNER JOIN只合并与其他查询结果匹配的记录。