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[数据库]求字符串中最大的递增子序列


数据库环境:SQL SERVER 2005

如题,求字符串“abcbklmnodfghijkmer”中最大的递增子序列。这个字符串有点特别,

只由26个小写字母a-z组成。

大概思路如下:

1.将字符串转到一列存储,并生成行号

2.设置一个递增计数器列,默认为1,比较上下行的字符,如果在字典中的顺序是递增,

则计数器加1,否则,计数器置1

3.找出计数器最大的数及对应的行号,根据这2个数截取字符串

 

思路有了,下面直接贴代码

DECLARE @vtext VARCHAR(255)SET @vtext = 'abcbklmnodfghijkmer'/*讲字符串转成一列存储,并生成行号*/WITH  x0     AS ( SELECT  number AS id ,            SUBSTRING(@vtext, number, 1) AS letter        FROM   master.dbo.spt_values        WHERE  type = 'P'            AND number <= LEN(@vtext)            AND number >= 1       ),/*实现计数器*/    x1 ( id, letter, clen )     AS ( SELECT  id ,            letter ,            1 AS clen        FROM   x0        WHERE  id = 1        UNION ALL        SELECT  x0.id ,            x0.letter ,            CASE WHEN x1.letter <= x0.letter THEN x1.clen + 1               ELSE 1            END AS clen        FROM   x0 ,            x1        WHERE  x0.id = x1.id + 1       )  /*截取字符串*/  SELECT SUBSTRING(@vtext, start, sublen) AS 最大子序列  FROM  ( SELECT  id ,            clen ,            MAX(clen) OVER ( ) AS maxclen ,            id - MAX(clen) OVER ( ) + 1 AS start ,            MAX(clen) OVER ( ) AS sublen       FROM   x1      ) t  WHERE  clen = maxclen

求出的最大子序列是

(本文完)