sql update 特殊用法

1.Count 更新

  schoolbasicinfo 中存在 MENBERCOUNT 字段记录条数  vw_shcoolbasic 做Count查询

update  schoolbasicinfo  set  MENBERCOUNT =
( select  vw_shcoolbasic.countnum  from  vw_shcoolbasic  where  schoolbasicinfo.schoolinfoid = vw_shcoolbasic.schoolid)

 

2.case 更新

 

代码

   update  websitequestion
       set  hasanswered = case
       when  hasanswered = ' n '   then   ' 0 '
       else   ' 1 '   end  
  多个when
   update  websitequestion
       set  hasanswered = case
       when  hasanswered = ' n '   then   ' 0 '
       when  hasanswered = ' y '   then   ' 1 '
       else   ' 0 '   end  

 

1.26 补充：在count更新的时候，可能schoolbasicinfo.schoolinfoid 不存在于vw_shcoolbasic 

这个时候我们更新schoolbasicinfo想得到的MENBERCOUNT为0，可实际schoolbasicinfo得到的却是 null

可以结合case

代码

  update  schoolbasicinfo  set  MENBERCOUNT =
       ( 
         case   when  schoolinfoid  not   in  ( select  schoolid  from  vw_shcoolbasic  group   by  schoolid)  then   ' 0 '
         else   ( select  vw_shcoolbasic.countnum  from  vw_shcoolbasic  where  schoolbasicinfo.schoolinfoid = vw_shcoolbasic.schoolid)
         end
        )