自然連線

• 概念

  • 自動判斷條件連線，判斷的條件是依據同名欄位

• 小結

  • 表連線是通過同名欄位來連線的
  • 如果沒有同名欄位就返回笛卡爾積
  • 同名的連線欄位只顯示一個,並且將該欄位放在最前面

自然內連線(natural join)

MariaDB [sel]> select * from grades natural join resume;
+-------+---------+------+----+-----------+
| name  | chinese | math | id | skill     |
+-------+---------+------+----+-----------+
| Sunny |      93 |   96 |  1 | php       |
| Jerry |      97 |   91 |  3 | php,mysql |
+-------+---------+------+----+-----------+
# `2 rows in set (0.023 sec)`

MariaDB [sel]> select * from grades;
+-------+---------+------+
| name  | chinese | math |
+-------+---------+------+
| Sunny |      93 |   96 |
| Jerry |      97 |   91 |
| Marry |      95 |   94 |
| Tommy |      98 |   94 |
+-------+---------+------+
# `4 rows in set (0.000 sec)`

MariaDB [sel]> select * from resume;
+----+-------+-----------+
| id | name  | skill     |
+----+-------+-----------+
|  1 | Sunny | php       |
|  2 | Kimmy | php       |
|  3 | Jerry | php,mysql |
+----+-------+-----------+
# `3 rows in set (0.000 sec)`
 

自然左外連線(natural left join)

MariaDB [sel]> select * from grades natural left join resume;
+-------+---------+------+------+-----------+
| name  | chinese | math | id   | skill     |
+-------+---------+------+------+-----------+
| Sunny |      93 |   96 |    1 | php       |
| Jerry |      97 |   91 |    3 | php,mysql |
| Marry |      95 |   94 | NULL | NULL      |
| Tommy |      98 |   94 | NULL | NULL      |
+-------+---------+------+------+-----------+
# `4 rows in set (0.001 sec)`
 

自然右外連線(natural right join)

MariaDB [sel]> select * from grades natural right join resume;
+-------+----+-----------+---------+------+
| name  | id | skill     | chinese | math |
+-------+----+-----------+---------+------+
| Sunny |  1 | php       |      93 |   96 |
| Jerry |  3 | php,mysql |      97 |   91 |
| Kimmy |  2 | php       |    NULL | NULL |
+-------+----+-----------+---------+------+
# `3 rows in set (0.000 sec)`