SQL存在一个表而不在另一个表中的数据

select a.id,a.oacode,a.custid,a.custname,a.xsz,a.salename,a.communicationtheme,a.communicationproperty,a.communicationtime,a.productmanager,'' creator,a.creator oaCreator,a.createdate,case when a.oastate='正常结束' then 3 else 0 end status from reception_flow a
where  (1=1)
and (a.oastate='正常结束' or a.oastate='反馈中' or a.oastate='已反馈')
and (select count(1) as num from technology_support s where s.oacode=a.oacode)=0