为什么同一行显示2次？

Question

SELECT patient.P_ID, patient.P_NAME, patient.P_AGE, 
       patient.P_GENDER, patient.ADDRESS, patient.DOA, 
       patient.P_ID DIAGNOSE
FROM patient,presciption
WHERE patient.P_ID != presciption.P_ID

Answer 1

只要在预告表中找不到患者，请使用NOT EXISTS返回患者：

select patient.P_ID, patient.P_NAME, patient.P_AGE, patient.P_GENDER, patient.ADDRESS, patient.DOA, patient.P_ID DIAGNOSE
from patient
where not exists (select 1 from presciption
                  where patient.P_ID = presciption.P_ID)