这篇文章主要介绍“MySQL内连接和外连接有哪些区别”，在日常操作中，相信很多人在mysql内连接和外连接有哪些区别问题上存在疑惑，小编查阅了各式资料，整理出简单好用的操作方法，希望对大家解答”mysql内连接和外连接有哪些区别”的疑惑有所帮助！接下来，请跟着小编一起来学习吧！
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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
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)
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)
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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