将字符串中的重复字符剔除

数据库环境：SQL SERVER2008R2

在网上看到一网友提的需求，要求把字符串列中的重复字符剔除，只保留一个。我简单的把需求描述下，

比如，有一个t表，A1列存字符串，存储的内容如下：

A1

A,B,B,C

C,C,D

F,S,S

剔除重复字符后的结果如下：

A1

A,B,C

C,D

F,S

思路：每一行记录生成一个行号，把字符串中的字符全部存到一列，根据行号和字符去重，然后再用FOR XML PATH合并到一行

/*数据准备*/
WITH    x0
          AS ( SELECT   1 AS id ,
                        'A,B,B,C' AS A1
               UNION ALL
               SELECT   2 AS id ,
                        'C,C,D' AS A1
               UNION ALL
               SELECT   3 AS id ,
                        'F,S,S' AS A1
             ),/*将所有字符转存一列，去重*/
        x2
          AS ( SELECT  DISTINCT
                        a.id ,
                        SUBSTRING(a.A1, b.number,
                                  CHARINDEX(',', a.A1 + ',', b.number)
                                  - b.number) AS A1
               FROM     x0 a ,
                        master..spt_values b
               WHERE    b.number >= 1
                        AND b.number <= LEN(a.A1)
                        AND b.type = 'P'
                        AND SUBSTRING(',' + a.A1, b.number, 1) = ','
             )
     /*根据原先的行号，把行号相同的转回到一行上*/
    SELECT  LEFT(A1, LEN(A1) - 1) AS A1
    FROM    ( SELECT    id ,
                        ( SELECT    a.A1 + ','
                          FROM      x2 a
                          WHERE     a.id = b.id
                        FOR
                          XML PATH('')
                        ) AS A1
              FROM      x2 b
              GROUP BY  id
            ) t

SQL比较好理解，并加了一些注释，这里不再重复。

(全文完)