Mysql，Case When,Case多個欄位

 select distinct a.PatientID,a.PatientCode,a.PatientSex,a.MobileNo,a.HomePhoneNo,a.UserAge,a.PatientName,a.PatientIDCard,  DATE_FORMAT(a.RegistDate,'%Y-%m-%d') as RegistDate, 
 case  when b.usedstartTime is not null and b.UsedEndTime is null then '1'
when b.usedstartTime is not null and b.UsedEndTime is not  null then '2'
 end  as 'usedState'
 from mets_v_patient_baseinfo  a
 
 left join mets_devices_used_history b on a.patientid = b.PatientID
 
 where   (select ifnull(IsDeleted,0) from userpublic_info where UserID = a.PatientID ) = 0
 and  1=1 
 order by PatientID Desc limit 0,15