本教程操作环境：windows7系统、mysql8版本、Dell G3电脑。
区别内连接（inner join）：取出连接表中匹配到的数据，匹配不到的不保留
外连接（outer join）：取出连接表中匹配到的数据，匹配不到的也会保留，其值为NULL
示例表
users表
mysql> select * from users;+----+-------+| id | name |+----+-------+| 1 | john || 2 | May || 3 | Lucy || 4 | Jack || 5 | James |+----+-------+5 rows in set (0.00 sec)
topics表
mysql> select * from topics;+----+---------------------------------------+---------+| id | title | user_id |+----+---------------------------------------+---------+| 1 | Hello world | 1 || 2 | PHP is the best language in the world | 2 || 3 | Laravel artist | 6 |+----+---------------------------------------+---------+3 rows in set (0.00 sec)内连接(inner join)
示例
mysql> select * from users as u inner join topics as t on u.id=t.user_id;+----+------+----+---------------------------------------+---------+| id | name | id | title | user_id |+----+------+----+---------------------------------------+---------+| 1 | john | 1 | Hello world | 1 || 2 | May | 2 | PHP is the best language in the world | 2 |+----+------+----+---------------------------------------+---------+2 rows in set (0.00 sec)
inner可以省略，as是给表起别名，也可以省略
mysql> select * from users u join topics t on u.id=t.user_id;+----+------+----+---------------------------------------+---------+| id | name | id | title | user_id |+----+------+----+---------------------------------------+---------+| 1 | john | 1 | Hello world | 1 || 2 | May | 2 | PHP is the best language in the world | 2 |+----+------+----+---------------------------------------+---------+2 rows in set (0.00 sec)
以上两句等价于
mysql> select * from users,topics where users.id=topics.user_id;+----+------+----+---------------------------------------+---------+| id | name | id | title | user_id |+----+------+----+---------------------------------------+---------+| 1 | john | 1 | Hello world | 1 || 2 | May | 2 | PHP is the best language in the world | 2 |+----+------+----+---------------------------------------+---------+2 rows in set (0.00 sec)外连接(outer join)
左外连接（left outer join）：以左边的表为主表
右外连接（right outer join）：以右边的表为主表
以某一个表为主表，进行关联查询，不管能不能关联的上，主表的数据都会保留，关联不上的以NULL显示
通俗解释就是：先拿出主表的所有数据，然后到关联的那张表去找有没有符合关联条件的数据，如果有，正常显示，如果没有，显示为NULL
示例
mysql> select * from users as u left join topics as t on u.id=t.user_id;+----+-------+------+---------------------------------------+---------+| id | name | id | title | user_id |+----+-------+------+---------------------------------------+---------+| 1 | john | 1 | Hello world | 1 || 2 | May | 2 | PHP is the best language in the world | 2 || 3 | Lucy | NULL | NULL | NULL || 4 | Jack | NULL | NULL | NULL || 5 | James | NULL | NULL | NULL |+----+-------+------+---------------------------------------+---------+5 rows in set (0.00 sec)
等价于以下，只是字段的位置不一样
mysql> select * from topics as t right join users as u on u.id=t.user_id;+------+---------------------------------------+---------+----+-------+| id | title | user_id | id | name |+------+---------------------------------------+---------+----+-------+| 1 | Hello world | 1 | 1 | john || 2 | PHP is the best language in the world | 2 | 2 | May || NULL | NULL | NULL | 3 | Lucy || NULL | NULL | NULL | 4 | Jack || NULL | NULL | NULL | 5 | James |+------+---------------------------------------+---------+----+-------+5 rows in set (0.00 sec)
左外连接和右外连接是相对的，主要就是以哪个表为主表去进行关联
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