子查询更新Update字段值

在进行数据修复时，遇到一个问题一直没有怎么注意的问题。

 update wf_task wf
   set ETime =
   (select NewEtime
      from wf_task_Mod_backup_20171016 a
     where wf.Entgid = a.EntGid
       and wf.Flowgid = a.FlowGid  
       and wf.taskGid = a.taskGid
       and wf.execcode = a.execcode 
       and wf.code = a.code 
       and wf.btime = a.btime) 

以上执行的结果会让匹配的数据更新为目标数据，但是不匹配的都会被更新为null

 update wf_task wf
   set ETime =
   (select NewEtime
      from wf_task_Mod_backup_20171016 a
     where wf.Entgid = a.EntGid
       and wf.Flowgid = a.FlowGid  
       and wf.taskGid = a.taskGid
       and wf.execcode = a.execcode 
       and wf.code = a.code 
       and wf.btime = a.btime) 
where exists (select 1
      from wf_task_Mod_backup_20171016
     where Entgid = wf.Entgid
       and FlowGid = wf.flowgid
       and TaskGid = wf.Taskgid);

所以要在后面加上exists，限制数据更新的范围。切忌update 表 set = （字段）后面没有接where 条件。