有点晕的SQL语句

一个表（PW_二氧化硫收费标准），数据类型如下：



表中数据如下：

 1         1900-1-1          2004-7-1          .2
 2         2004-7-1          2005-7-1          .4
 3         2005-7-1          2008-11-1        .6

表中数据代表不同时期内二氧化硫不同的收费标准

要求根据输入的年月（格式如"200501"）查询出当前年月的二氧化硫收费标准

SQL语句如下：

                            SELECT 
                                收费标准
                            FROM 
                                (
                                    SELECT 
                                        CAST
                                            (
                                                CAST(YEAR(开始时间) AS CHAR(4)) + 
                                                    (    
                                                        CASE 
                                                            LEN(CAST(MONTH(开始时间) AS VARCHAR(2))) 
                                                        WHEN 
                                                            1 
                                                        THEN 
                                                            '0' + CAST(MONTH(开始时间) AS VARCHAR(2)) 
                                                        ELSE 
                                                            CAST(MONTH(开始时间) AS VARCHAR(2)) 
                                                        END
                                                    ) 
                                                AS INT
                                            ) 
                                        AS 
                                        开始时间年和月份, 
                                        CAST
                                            (
                                                CAST(YEAR(结束时间) AS CHAR(4)) + 
                                                    (
                                                        CASE 
                                                            LEN(CAST(MONTH(结束时间) AS VARCHAR(2))) 
                                                        WHEN 
                                                            1 
                                                        THEN
                                                            '0' + CAST(MONTH(结束时间) AS VARCHAR(2)) 
                                                        ELSE 
                                                            CAST(MONTH(结束时间) AS VARCHAR(2)) 
                                                        END
                                                    ) 
                                                AS INT
                                            ) 
                                        AS 
                                        结束时间年和月份,
                                        收费标准
                                FROM 
                                    [PW_二氧化硫收费标准]
                            )
                             二氧化硫收费标准转换
                        WHERE 
                                (二氧化硫收费标准转换.开始时间年和月份 <= '200501') 
                            AND 
                                (二氧化硫收费标准转换.结束时间年和月份 > '200501')

其实设计数据库的时候把表结构和其中的数据考虑周全点，就不用这么麻烦啦，呵呵……