求大神SQL2008r2语句优化

SQL server 2008r2

现在日常操作是查询gfdsa表内的数据按日期保存到另外一个表的（为了后面去重用）

现在查询一次需要1分钟左右，怎么才优化到30秒以内，或者有什么好的思路

use ccc

 select  t.*    

     from

     (select aa,bb,cc,dd, row_number() 

     over (partition by bb order by bb desc)

     rn 

     from (

          select aa

          =CONVERT(nvarchar(max),DECRYPTBYKEY(aa))

          , bb ,cc ,dd    from   ccc..[gfdsa]except 

           select aa, bb ,cc ,dd     from   rfv..[qaw711]except

           select aa, bb ,cc ,dd     from   rfv..[qaw712]except

           select aa, bb ,cc ,dd     from   rfv..[qaeq717]except

           select aa, bb ,cc ,dd     from   rfv..[qase702]except

           select aa, bb ,cc ,dd     from   rfv..[qase703]except

           select aa, bb ,cc ,dd     from   rfv..[qase704]except

           select aa, bb ,cc ,dd     from   rfv..[qase705]except

           select aa, bb ,cc ,dd     from   rfv..[qase708] except

           select aa, bb ,cc ,dd     from   rfv..[qase709] except

           select aa, bb ,cc ,dd     from   rfv..[qase710] except

           select aa, bb ,cc ,dd     from   rfv..[qase711] except

           select aa, bb ,cc ,dd     from   rfv..[qase716] except

           select aa, bb ,cc ,dd     from   rfv..[qase718] except

           select aa, bb ,cc ,dd     from   rfv..[qase719] except

           select aa, bb ,cc ,dd     from    rfv..[qzew07yc] except

           select aa, bb ,cc ,dd     from    rfv..[qzsew710]except

           select aa, bb ,cc ,dd     from    rfv..[qzsew711]except

           select aa, bb ,cc ,dd     from    rfv..[qzsew712]except

           select aa, bb ,cc ,dd     from    rfv..[qzsew713]except

           select aa, bb ,cc ,dd     from    rfv..[qzsew715]except

           select aa, bb ,cc ,dd     from    rfv..[qzsew716]except

           select aa, bb ,cc ,dd     from    rfv..[qzsew720]except

           select aa, bb ,cc ,dd     from   tgb..[qzrdx713]except

           select aa, bb ,cc ,dd     from   tgb..[qxdes711]except

           select aa, bb ,cc ,dd     from   tgb..[qzxswa718]except

           select aa, bb ,cc ,dd     from   tgb..[qxswaz711]except

           select aa, bb ,cc ,dd     from   tgb..[qxswaz716]except

           select aa, bb ,cc ,dd     from   tgb..[qxswzs711]except

           select aa, bb ,cc ,dd     from   tgb..[qcdesx702]except

           select aa, bb ,cc ,dd     from   tgb..[qwsxcsxz715]except

           select aa, bb ,cc ,dd     from   tgb..[qwsxcsxz717]except

           select aa, bb ,cc ,dd     from   tgb..[qtxdw709]except

           select aa, bb ,cc ,dd     from   tgb..[qywsxa704]except

           select aa, bb ,cc ,dd     from   tgb..[quwsx627]except

           select aa, bb ,cc ,dd     from   tgb..[qixse708]except

           select aa, bb ,cc ,dd     from   tgb..[qoxde715]except

           select aa, bb ,cc ,dd     from   tgb..[qoxde716]except

           select aa, bb ,cc ,dd     from   tgb..[qoxde717]except

           select aa, bb ,cc ,dd     from   tgb..[qpswxa715]except

           select aa, bb ,cc ,dd     from   yhn..[qlwsxe2]except

           select aa, bb ,cc ,dd     from   yhn..[qkxdes718]except

           select aa, bb ,cc ,dd     from   ujm..[qjxdre626]except

           select aa, bb ,cc ,dd     from   ujm..[qhedxs710]except

           select aa, bb ,cc ,dd     from   ujm..[qhedxs715]except

           select aa, bb ,cc ,dd     from   ujm..[qhedxs717]except

           select aa, bb ,cc ,dd     from   okm..[qhygvd718]except

           select aa, bb ,cc ,dd     from   ijn..[qwsngf]except

           select aa, bb ,cc ,dd     from   ijn..[qfwsxcd712]except

           select aa, bb ,cc ,dd     from   ijn..[qfwsxcd715]except

           select aa, bb ,cc ,dd     from   ijn..[wqaszx711]except

           select aa, bb ,cc ,dd     from   ijn..[wsxcde709]except

           select aa, bb ,cc ,dd     from   ijn..[wbgtfvd705]except

           select aa, bb ,cc ,dd     from   ijn..[wsxaqzde710]except

           select aa, bb ,cc ,dd     from   ijn..[wsxaqzde711]except

           select aa, bb ,cc ,dd     from   ijn..[wsxaqzde719]except

           select aa, bb ,cc ,dd     from   ijn..[wedcftv708]except

           select aa, bb ,cc ,dd     from   ijn..[wedcftv715]except

           select aa, bb ,cc ,dd     from   ijn..[eswqazxs710]except

           select aa, bb ,cc ,dd     from   ijn..[esxwdcz627]except

           select aa, bb ,cc ,dd     from   ijn..[rwsxcdeq705]except

           select aa, bb ,cc ,dd     from   ijn..[rwsxcdeq708]except

           select aa, bb ,cc ,dd     from   ijn..[rwsxcdeq708s?]except

           select aa, bb ,cc ,dd     from   ijn..[rwsxcdeq709]except

           select aa, bb ,cc ,dd     from   ijn..[rwsxcdeq711]except

           select aa, bb ,cc ,dd     from   ijn..[rwsxcdeq712]except

           select aa, bb ,cc ,dd     from   ijn..[rwsxcdeq713]except

           select aa, bb ,cc ,dd     from   ijn..[rwsxcdeq716]except

           select aa, bb ,cc ,dd     from   ijn..[rwsxcdeq718]except

           select aa, bb ,cc ,dd     from   uhb..[tfrcvcde]except

           select aa, bb ,cc ,dd     from   uhb..[ygbtrfvcd]except

           select aa, bb ,cc ,dd     from   uhb..[uhnbecd712]




     )   as   ss

     ) t

  where   rn<=1000 and

  bb  in('asd')

select distinct 某列 from (table1 union table2 union table 3)

不太理解单一列去重是什么含义，是指只获取mdn唯一的几个记录，并用其他条件再筛选吗？这样的话是否考虑用分组获取唯一的mdn作为条件记录到一个表里（结构：表名、mdn），然后再以这个表关联或作为条件，不知是否可以满足需求

https://www.cnblogs.com/kevin1982/p/4181455.html

通过SQLSERVER数据集合的交、并、差集运算(intersect,union,except)完成相关主要功能