sql - 连接按列分组的值

Question

我有下表:

Code    Ref     Value
A1      Car     A
A1      Car     -
A1      Car     B
B2      Truck   CC
B2      Truck   D
B2      Truck   -
C3      Van     E
C3      Van     F
C3      Van     -
C3      Van     G

我试图实现的目标是一个将所有值分组在一起的连接字符串，如下所示:

Code   Ref    Value
A1     Car    A-B
B2     Truck  CCD-
C3     Van    EF-G

我离开了这个例子here ，但无处可去。这是我想到的:

SELECT [Table].[Code]
, [Table].[Ref]
, STUFF((SELECT DISTINCT [Value]
  FROM [Table2]
  FOR XML PATH ('')),1, 1,'') AS Values
FROM [Table]
LEFT JOIN [Table2] ON
[Table2].[Code] = [Table].[Code]

我哪里出错了？有没有更有效的方法来做到这一点？

Answer 1

您没有任何内容将您的内部和外部引用链接到 [Table] ，并且您还需要使外部引用与众不同。最后，您需要在子查询中没有列名，或者需要是 [text()]

SELECT  [Code]
        ,[Ref]
        ,STUFF((SELECT DISTINCT [Value] AS [text()]
                FROM [Table] AS T2
                WHERE T1.Code = T2.Code -- LINK HERE
                AND T2.Ref = T2.Ref     -- AND HERE
                FOR XML PATH ('')
                ),1, 1,'') AS [Values]
FROM    [Table] AS T1
GROUP BY T1.Code, T1.Ref; -- GROUP BY HERE

顺便说一句，您不需要使用 STUFF因为您没有分隔符，STUFF通常用于从字符串的开头删除所选的分隔符。所以当你有一个类似 ,value1,value2,value3 的字符串时, STUFF(string, 1, 1, '')将用 '' 替换第一个字符留给你 value1,value2,value3 .

您还应该使用value xquery 方法来确保您不会被特殊字符绊倒，如果您不这样做并且尝试连接 ">>"和"<<"你最终不会得到 ">><<"如您所愿，您会得到 ">><<" ，所以更好的查询是:

SELECT  t1.Code,
        t1.Ref,
        [Values] = (SELECT  DISTINCT [text()] = [Value]
                    FROM    [Table] AS t2
                    WHERE   T1.Code = T2.Code
                    AND     T2.Ref = T2.Ref
                    FOR XML PATH (''), TYPE
                ).value('.', 'NVARCHAR(MAX)')
FROM    [Table] AS T1
GROUP BY t1.Code, t1.Ref;

---

附录

根据对问题的最新编辑，看起来好像您的 Value列来自另一个表，通过 Code 链接到第一个表。如果有的话，这会让您的查询更简单。您不需要JOIN ，但您仍然需要确保有一个表达式将外部表链接到子查询的内部表。我假设第一个表中的行是唯一的，因此您可能不需要分组依据:

SELECT  t1.Code,
        t1.Ref,
        [Values] = (SELECT  DISTINCT [text()] = t2.[Value]
                    FROM    [Table2] AS t2
                    WHERE   T1.Code = T2.Code
                    FOR XML PATH (''), TYPE
                ).value('.', 'NVARCHAR(MAX)')
FROM    [Table] AS T1;

---

工作示例

CREATE TABLE #Table1 (Code CHAR(2), Ref VARCHAR(10));
INSERT #Table1 VALUES ('A1', 'Car'), ('B2', 'Truck'), ('C3', 'Van');

CREATE TABLE #Table2 (Code CHAR(2), Value VARCHAR(2));
INSERT #Table2
VALUES ('A1', 'A'), ('A1', '-'), ('A1', 'B'),
        ('B2', 'CC'), ('B2', 'D'), ('B2', '-'),
        ('C3', 'F'), ('C3', '-'), ('C3', 'G');

SELECT  t1.Code,
        t1.Ref,
        [Values] = (SELECT  DISTINCT [text()] = t2.[Value]
                    FROM    #Table2 AS t2
                    WHERE   T1.Code = T2.Code
                    FOR XML PATH (''), TYPE
                ).value('.', 'NVARCHAR(MAX)')
FROM    #Table1 AS T1;