mysql内连接和外连接有哪些区别

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mysql内连接和外连接的区别：内连接会取出连接表中匹配到的数据，匹配不到的不保留；而外连接会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。

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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