mysql內連線和外連線有什麼差別

mysql內連接和外連接的區別：內連接會取出連接表中匹配到的數據，匹配不到的不保留；而外連接會取出連接表中匹配到的數據，匹配不到的也會保留，其值為NULL。

本教學操作環境：windows7系統、mysql8版本、Dell G3電腦。

區別

• 內連接（inner join）：取出連接表中匹配到的數據，符合不到的不保留
• 外連接（outer join） ：取出連接表中符合的數據，符合不到的數據也會保留，其值為NULL

#範例表

##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)

內連接(inner join)

範例

• 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是給表起別名，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)

以上兩句等價於

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)

外連接(outer join)

左外連接（left outer join）：以左邊的表為主表
• 右外連接（right outer join）：以右邊的表為主表

以某一個表為主表，進行關聯性查詢，不管能不能關聯的上，主表的資料都會保留，關聯不上的以NULL顯示

通俗解釋就是：先拿出主表的所有數據，然後到關聯的那張表去找有沒有符合關聯條件的數據，如果有，正常顯示，如果沒有，顯示為NULL

範例

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)

等價於以下，只是字段的位置不一樣

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)

左外連接與右外連接是相對的，主要就是以哪個表為主表去進行關聯

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