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

  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

 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
        )