https://blog.csdn.net/babyfish13/article/details/78082844

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样式1：

update xcs_user_credit_score a1,xcs_user_credit_score a2
   set a1.user_currday_score=a1.user_currday_increment_score+a2.user_currday_score
 where a1.pt_day='2017-09-20' and a2.pt_day='2017-09-19'
   and a1.uid=a2.uid;

样式2：

update xcs_user_credit_score a1
     inner join xcs_user_credit_score a2
        on a1.uid=a2.uid
       set a1.user_currday_score=a1.user_currday_increment_score+a2.user_currday_score
     where a1.pt_day='2017-09-20' and a2.pt_day='2017-09-19'
    ;

**突然发现mysql的一些sql语法也很神奇，像这样式的sql语句，在oracle中是行不通的，需要其他的形式。
**另外的几个例子：

--sub:
    update xcs_user_credit_score a1,xcs_user_credit_score a2
       set a1.user_currday_score=(case when a2.user_currday_score-10>0 then a2.user_currday_score-10 else 0 end)
     where a1.pt_day='2017-09-20' and a2.pt_day='2017-09-19'
       and a1.uid=a2.uid
       and (a1.seqing_score=0 and a1.user_silent_score=0 and a1.jubao_score=0 and a1.chengpaopao_score=0 and a1.weifan_score=0 and a1.upper_manage_score=0 and a1.pay_active_score=0 and a1.view_active_score=0 and a1.message_active_score=0 and a1.gift_active_score=0 and a1.live_active_score=0)
       and a2.user_currday_score>0
    ;
     
    --plus:
    update xcs_user_credit_score a1,xcs_user_credit_score a2
       set a1.user_currday_score=(case when a1.user_currday_score+10<0 then a1.user_currday_score+10 else 0 end)
     where a1.pt_day='2017-09-20' and a2.pt_day='2017-09-19'
       and a1.uid=a2.uid
       and (a1.seqing_score=0 and a1.user_silent_score=0 and a1.jubao_score=0 and a1.chengpaopao_score=0 and a1.weifan_score=0)
       and a1.user_currday_score<0
    ;