mysql/mariadb学习记录——连接查询（JOIN）

//本文使用的数据表格
//persons表中id_p为主键
//orders表中id_o为主键，id_p为外键参考persons表中的id_p

mysql> select * from persons;select * from orders;
+------+----------+-----------+----------------+----------+
| id_p | lastname | firstname | address        | city     |
+------+----------+-----------+----------------+----------+
|    1 | Adams    | John      | Oxford Street  | London   |
|    2 | Bush     | George    | Fifth Avenue   | New York |
|    3 | Carter   | Thomas    | Changan Street | Beijing  |
+------+----------+-----------+----------------+----------+
3 rows in set (0.05 sec)

+------+---------+------+
| id_o | orderNo | id_p |
+------+---------+------+
|    1 | 77895   |    3 |
|    2 | 44678   |    3 |
|    3 | 22456   |    1 |
|    4 | 24562   |    1 |
|    5 | 34764   |   65 |
+------+---------+------+
5 rows in set (0.05 sec)

1.内连接 [inner] join：

　　内连接也叫连接，或者自然连接：

//inner可删
mysql> select lastname,firstname,orderNo from persons inner join orders on persons.id_p=orders.id_p order by lastname;
+----------+-----------+---------+
| lastname | firstname | orderNo |
+----------+-----------+---------+
| Adams    | John      | 24562   |
| Adams    | John      | 22456   |
| Carter   | Thomas    | 77895   |
| Carter   | Thomas    | 44678   |
+----------+-----------+---------+

//使用where达到同样的效果
mysql> select lastname,firstname,orderNo from persons,orders where persons.id_p=orders.id_p;
+----------+-----------+---------+
| lastname | firstname | orderNo |
+----------+-----------+---------+
| Carter   | Thomas    | 77895   |
| Carter   | Thomas    | 44678   |
| Adams    | John      | 22456   |
| Adams    | John      | 24562   |
+----------+-----------+---------+

2.左连接 left [outer] join：

　　先看左连接的语句与查询结果

//outer可删去
mysql> select lastname,firstname,orderNo from persons left join orders on persons.id_p=orders.id_p order by lastname;
+----------+-----------+---------+
| lastname | firstname | orderNo |
+----------+-----------+---------+
| Adams    | John      | 24562   |
| Adams    | John      | 22456   |
| Bush     | George    | NULL    |
| Carter   | Thomas    | 77895   |
| Carter   | Thomas    | 44678   |
+----------+-----------+---------+

　　通过这个结果我们可以发现lastname为Bush没有orderNo，而lastname，firstname属性是来自persons表，orderNo属性是来自orders表可见左连接的作用时把在left join 左边persons表格的所有信息都显示出来，即使是没有和left join右边的表 orders中字段相匹配的信息也显示出来，并且给orderNo的值填为NULL。

　　如果把left join两侧的 persons和 orders位置互换那么结果显示的就是left join 左边表orders表的所有信息，同时把未匹配到的left join右边表persons中的lastname与firstname值置为NULL，这个结果与未交换join两个表名位置的右连接的结果一致。

//将left join两侧的 persons和 orders位置互换
mysql> select lastname,firstname,orderNo from orders left join persons on persons.id_p=orders.id_p order by lastname;
+----------+-----------+---------+
| lastname | firstname | orderNo |
+----------+-----------+---------+
| NULL     | NULL      | 34764   |
| Adams    | John      | 24562   |
| Adams    | John      | 22456   |
| Carter   | Thomas    | 44678   |
| Carter   | Thomas    | 77895   |
+----------+-----------+---------+

3.右连接 right [outer] join:

//outer可删去
mysql> select lastname,firstname,orderNo from persons right outer join orders on persons.id_p=orders.id_p order by lastname;
+----------+-----------+---------+
| lastname | firstname | orderNo |
+----------+-----------+---------+
| NULL     | NULL      | 34764   |
| Adams    | John      | 22456   |
| Adams    | John      | 24562   |
| Carter   | Thomas    | 77895   |
| Carter   | Thomas    | 44678   |
+----------+-----------+---------+

　　同理与左连接，结果显示的就是right join 左边表persons表的所有信息，同时把未匹配到的right join右边表orders中的orderNo值置为NULL。

4.全连接full join（不过mysql/mariadb不支持）

 　　全连接则是把两个表的信息全部显示出来，无论两个表的信息匹配与否，未匹配上的信息都置为NULL。