例1 場景：有用戶表sys_user，用戶崗位關聯表 sys_user_post ，崗位表sys_post
SELECT * from sys_user t;
SELECT * from sys_user_post t;
SELECT * from sys_post t;
字段如下：
查詢用戶名稱以及該用戶的崗位名稱信息：
過程如下：
SELECT u.user_name,up.post_id from sys_user u LEFT JOIN sys_user_post up on u.user_id = up.user_id;
SELECT u.user_name,up.post_id,p.post_name from sys_user u LEFT JOIN sys_user_post up on u.user_id = up.user_id LEFT JOIN sys_post p on up.post_id = p.post_id;
結果：
場景2 用戶表(學生表、導師表或者員工與上級經理等)是同一張表sys_user，學生與導師關係對應表sys_user_tutor，表結構如下：
要求查詢出員工id，員工姓名，與該員工的導師id，導師姓名：
SELECT s.user_id as stuId, s.user_name as stuName,t.user_id as tutorId,t.user_name as tutorName FROM sys_user s LEFT JOIN sys_user_tutor st on s.user_id = st.user_id LEFT JOIN sys_user t on t.user_id = st.tutor_id;
查詢結果如下：
常用sql總結：
SELECT * from sys_user t;
SELECT * from sys_user_post t;
SELECT * from sys_post t;
SELECT * from sys_user_tutor t;
SELECT u.user_name,up.post_id from sys_user u LEFT JOIN sys_user_post up on u.user_id = up.user_id;
SELECT u.user_name,up.post_id,p.post_name from sys_user u LEFT JOIN sys_user_post up on u.user_id = up.user_id LEFT JOIN
sys_post p on up.post_id = p.post_id;
-- 按工號
SELECT * FROM user s LEFT JOIN student_tutor st on s.code = st.stuCode LEFT JOIN user t on st.tutorCode = t.code;
SELECT s.code as stuCode,s.name as stuName,t.code as tutorCode,t.name as tutorName FROM user s LEFT JOIN student_tutor st
on s.code = st.stuCode LEFT JOIN user t on st.tutorCode = t.code;
-- 按ID
SELECT * FROM user s LEFT JOIN student_tutor st on s.id = st.stuId LEFT JOIN user t on st.tutorId = t.id;
SELECT s.id as stuId, s.name as stuName,t.id as tutorId,t.name as tutorName FROM user s LEFT JOIN student_tutor st
on s.id = st.stuId LEFT JOIN user t on st.tutorId = t.id;
-- 實戰
SELECT s.user_id as stuId, s.user_name as stuName,t.user_id as tutorId,t.user_name as tutorName FROM sys_user s LEFT JOIN
sys_user_tutor st on s.user_id = st.user_id LEFT JOIN sys_user t on t.user_id = st.tutor_id;