mysql的join操作

join 用于多表中字段之间的联系，语法如下：

... FROM table1 INNER|LEFT|RIGHT JOIN table2 ON conditiona

table1:左表；table2:右表。

JOIN 按照功能大致分为如下三类：

INNER JOIN（内连接,或等值连接）：取得两个表中存在连接匹配关系的记录。

LEFT JOIN（左连接）：取得左表（table1）完全记录，无需关心右表（table2）中是否有对应匹配记录:有对应连接的记录会根据连接显示，没有响应的连接记录，补null

RIGHT JOIN（右连接）：与 LEFT JOIN 相反，取得右表（table2）完全记录，即使左表（table1）并无匹配对应记录。

注意：mysql不支持Full join,不过可以通过UNION 关键字来合并 LEFT JOIN 与 RIGHT JOIN来模拟FULL join.

接下来给出一个列子用于解释下面几种分类。如下两个表(A,B)

mysql> select A.id,A.name,B.name from A,B where A.id=B.id;
+----+-----------+-------------+
| id | name       | name             |
+----+-----------+-------------+
|  1 | Pirate       | Rutabaga      |
|  2 | Monkey    | Pirate            |
|  3 | Ninja         | Darth Vader |
|  4 | Spaghetti  | Ninja             |
+----+-----------+-------------+
4 rows in set (0.00 sec)

内连接，也叫等值连接，inner join产生同时符合A和B的一组数据。

mysql> select * from A inner join B on A.name = B.name;
+----+--------+----+--------+
| id | name   | id | name   |
+----+--------+----+--------+
|  1 | Pirate |  2 | Pirate |
|  3 | Ninja  |  4 | Ninja  |
+----+--------+----+--------+

Left join

mysql> select * from A left join B on A.name = B.name;
#或者：select * from A left outer join B on A.name = B.name;

+----+-----------+------+--------+
| id | name      | id   | name   |
+----+-----------+------+--------+
|  1 | Pirate    |    2 | Pirate |
|  2 | Monkey    | NULL | NULL   |
|  3 | Ninja     |    4 | Ninja  |
|  4 | Spaghetti | NULL | NULL   |
+----+-----------+------+--------+
4 rows in set (0.00 sec)

mysql> select * from A cross join B;
+----+-----------+----+-------------+
| id | name      | id | name        |
+----+-----------+----+-------------+
|  1 | Pirate    |  1 | Rutabaga    |
|  2 | Monkey    |  1 | Rutabaga    |
|  3 | Ninja     |  1 | Rutabaga    |
|  4 | Spaghetti |  1 | Rutabaga    |
|  1 | Pirate    |  2 | Pirate      |
|  2 | Monkey    |  2 | Pirate      |
|  3 | Ninja     |  2 | Pirate      |
|  4 | Spaghetti |  2 | Pirate      |
|  1 | Pirate    |  3 | Darth Vader |
|  2 | Monkey    |  3 | Darth Vader |
|  3 | Ninja     |  3 | Darth Vader |
|  4 | Spaghetti |  3 | Darth Vader |
|  1 | Pirate    |  4 | Ninja       |
|  2 | Monkey    |  4 | Ninja       |
|  3 | Ninja     |  4 | Ninja       |
|  4 | Spaghetti |  4 | Ninja       |
+----+-----------+----+-------------+
16 rows in set (0.00 sec)

#再执行：mysql> select * from A inner join B; 试一试

#在执行mysql> select * from A cross join B on A.name = B.name; 试一试