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