1 Join 类型有哪些

SQL Join 类型的区别

• Inner Join: 左,右表都有的数据
• Left Join: 左表返回所有的行, 右表没有的补充为 NULL
• Right Loin: 右表返回所有的行, 左表没有的补充为 NULL
• Full Outer Join: 左表 + 右表 所有数据

用户表:

答题表:

2 Inner Join

查询左表 和 右表都有的数据
用户表 user_no = 1000 和 答题表 user_no = 6543 , 不在交集范围, 没有在查询出来

-- inner join --
select tup.user_no, tup.university, tqpd.question_id, tqpd.result  
    from t_user_profile tup inner join t_question_practice_detail tqpd  
        on tup.user_no = tqpd.user_no;

3 Left Join

左表返回所有的行, 右表没有的补充为 NULL
用户表 user_no = 1000 对应的答题信息为空, 补充 NULL, 答题表 user_no = 6543 不在查询范围

-- left join --
select tup.user_no, tup.university, tqpd.question_id, tqpd.result  
    from t_user_profile tup left join t_question_practice_detail tqpd  
        on tup.user_no = tqpd.user_no;

4 Right Join

右表返回所有的行, 左表没有的补充为 NULL
答题表 user_no = 6543 对应的用户信息为空, 补充 NULL, 用户表 user_no = 1000 不在查询范围

-- right join --  
select tup.user_no, tup.university, tqpd.question_id, tqpd.result  
    from t_user_profile tup right join t_question_practice_detail tqpd  
        on tup.user_no = tqpd.user_no;

5 Full Join

在 MySQL 中没有 Full Join, 可以用 left join 和 right join 实现
用户表 user_no = 1000 对应的答题信息补充 NULL, 答题表 user_no = 6543 对应的用户信息补充 NULL

-- left join and right join --  
select tup.user_no, tup.university, tqpd.question_id, tqpd.result  
    from t_user_profile tup left join t_question_practice_detail tqpd  
        on tup.user_no = tqpd.user_no  
union  
select tup.user_no, tup.university, tqpd.question_id, tqpd.result  
    from t_user_profile tup right join t_question_practice_detail tqpd  
        on tup.user_no = tqpd.user_no;