Mysql的join语句

# group by 必须放在 order by 和 limit之前，不然会报错
# 你可以在 SELECT, UPDATE 和 DELETE 语句中使用 Mysql 的 JOIN 来联合多表查询。
# JOIN 常用分为如下三类(但不仅仅只有这三类)： 
# INNER JOIN（内连接,或等值连接）：获取两个表中字段匹配关系的记录；使用MySQL的INNER JOIN(也可以省略 INNER 使用 JOIN，效果一样)
# LEFT JOIN（左连接）：获取左表所有记录，即使右表没有对应匹配的记录。
# RIGHT JOIN（右连接）： 与 LEFT JOIN 相反，用于获取右表所有记录，即使左表没有对应匹配的记录。

select * from my_test_copy;  #第一张表

select * from my_test;  #第二张表

select a.id, a.name_people, b.name_adress from my_test_copy as a inner join my_test as b on a.name_adress=b.name_adress; #可用，或者用：（即不用 ‘as‘ 也行）
select a.id, a.name_people, b.name_adress from my_test_copy a inner join my_test b on a.name_adress=b.name_adress; #也可用

select a.id, a.name_people, a.name_adress from my_test a inner join my_test_copy b on a.name_adress=b.name_adress;   #针对于on的对应列进行笛卡尔积，返回所需要的列

select * from my_test a inner join my_test_copy b on a.name_adress=b.name_adress;  #相同列会自动加数字表示差异

select a.id, a.name_people, a.name_adress, b.id as b_id, b.name_people as name_people from my_test_copy a left join my_test b on a.name_adress=b.name_adress;

select * from my_test_copy a left join my_test b on a.name_adress=b.name_adress where b.name_adress is NULL; # 即left join中左边多余的部分

select a.id, a.name_people, a.name_adress, b.id as b_id, b.name_people as name_people from my_test_copy a right join my_test b on a.name_adress=b.name_adress;

select * from my_test_copy a right join my_test b on a.name_adress=b.name_adress where a.name_adress is NULL; # 即right join中左边多余的部分，此时该表为空。

# using 
select * from my_test_copy right join my_test using(name_adress);

select * from my_test_copy, my_test where my_test_copy.name_adress=my_test.name_adress;  # 这样也可以做到连接两个表

select * from my_test_copy a left join my_test b on a.name_adress=b.name_adress
union 
select * from my_test_copy a right join my_test b on a.name_adress=b.name_adress;