优化SQL小技巧：在适当的时候使用Union代替OR语句

由于OR语句会引起全表扫描，所以在很多时候OR将大大降低查询速度。在这种情况下就可以使用Union来替代OR实现“或”查询。

 原句：

SELECT   [ ID ]
      , [ UserID ]
      , [ Realname ]
      , [ Sex ]
      , [ Age ]
      , [ Stature ]
      , [ Nation ]
      , [ Nativeplace ]
      , [ Marry ]
      , [ Jobkind ]
      , [ Workexp ]
      , [ QQ ]
      , [ Tel ]
      , [ MobilePhone ]
      , [ Province ]
      , [ Area ]
      , [ City ]
      , [ HopePay ]
      , [ HopeCalling ]
      , [ HopeMetier ]
      , [ CanWorkTime ]
      , [ Culture ]
      , [ School ]
      , [ Specialty ]
      , [ Address ]
      , [ Abode ]
      , [ Email ]
      , [ Description ]
      , [ Hits ]
      , [ Secrecy ]
      , [ AddDateTime ]
      , [ ReflashTime ]
      , [ IsTop ]
      , [ CurrState ]
      , [ InBase ]
   FROM   [ SoXuData ] . [ dbo ] . [ tbJob_Resume ]   WHERE  Specialty = ' 计算机 '   OR  Culture = ' 大专 '  

使用Union的语句：

SELECT   [ ID ]
      , [ UserID ]
      , [ Realname ]
      , [ Sex ]
      , [ Age ]
      , [ Stature ]
      , [ Nation ]
      , [ Nativeplace ]
      , [ Marry ]
      , [ Jobkind ]
      , [ Workexp ]
      , [ QQ ]
      , [ Tel ]
      , [ MobilePhone ]
      , [ Province ]
      , [ Area ]
      , [ City ]
      , [ HopePay ]
      , [ HopeCalling ]
      , [ HopeMetier ]
      , [ CanWorkTime ]
      , [ Culture ]
      , [ School ]
      , [ Specialty ]
      , [ Address ]
      , [ Abode ]
      , [ Email ]
      , [ Description ]
      , [ Hits ]
      , [ Secrecy ]
      , [ AddDateTime ]
      , [ ReflashTime ]
      , [ IsTop ]
      , [ CurrState ]
      , [ InBase ]
   FROM   [ SoXuData ] . [ dbo ] . [ tbJob_Resume ]   WHERE  Specialty = ' 计算机 '  
     Union
     SELECT   [ ID ]
      , [ UserID ]
      , [ Realname ]
      , [ Sex ]
      , [ Age ]
      , [ Stature ]
      , [ Nation ]
      , [ Nativeplace ]
      , [ Marry ]
      , [ Jobkind ]
      , [ Workexp ]
      , [ QQ ]
      , [ Tel ]
      , [ MobilePhone ]
      , [ Province ]
      , [ Area ]
      , [ City ]
      , [ HopePay ]
      , [ HopeCalling ]
      , [ HopeMetier ]
      , [ CanWorkTime ]
      , [ Culture ]
      , [ School ]
      , [ Specialty ]
      , [ Address ]
      , [ Abode ]
      , [ Email ]
      , [ Description ]
      , [ Hits ]
      , [ Secrecy ]
      , [ AddDateTime ]
      , [ ReflashTime ]
      , [ IsTop ]
      , [ CurrState ]
      , [ InBase ]
   FROM   [ SoXuData ] . [ dbo ] . [ tbJob_Resume ]   WHERE  Culture = ' 大专 '

但是不是什么时候都适合这样做，如果“或”条件的是相同字段则使用OR语句效率要比较高，原因未明，有空再查资料