MySql左连接的使用

### 左连接会先查询出所有链接的表数据，之后再匹配WHERE后面的条件：
SELECT u.name,u.id AS userId,u.gender,u.birthday,u.mobile,u.img,p.disease_info,pb.sn,pb.state
    FROM (`user` u,`doctor_patient` d)
    LEFT JOIN `patient_disease_info` p ON d.patient_user_id = p.user_id
    LEFT JOIN patient_blood_pressure_bind_his pb ON d.patient_user_id = pb.user_id AND pb.deleted = 0
    WHERE d.doctor_user_id = 'dffb19e2ebb94243b0275ce2b18482c0' AND u.id = d.patient_user_id AND 
    d.deleted = 0 AND  u.deleted = 0

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### 查出所有数据之后，可以使用AND匹配最终字段的数据情况：
SELECT u.name,u.id AS userId,u.gender,u.birthday,u.mobile,u.img,p.disease_info,pb.sn,pb.state
    FROM (`user` u,`doctor_patient` d)
    LEFT JOIN `patient_disease_info` p ON d.patient_user_id = p.user_id
    LEFT JOIN patient_blood_pressure_bind_his pb ON d.patient_user_id = pb.user_id AND pb.deleted = 0
    WHERE d.doctor_user_id = 'dffb19e2ebb94243b0275ce2b18482c0' AND u.id = d.patient_user_id AND 
    d.deleted = 0 AND  u.deleted = 0 AND pb.state IS NOT NULL

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