sql语句，左、右、内连接之间的区别

1. left join(左联接)： 返回包括左表中的所有记录和右表中联结字段相等的记录。

   
   

   right join(右联接)： 返回包括右表中的所有记录和左表中联结字段相等的记录。

   
   

   inner join(等值连接)： 只返回两个表中联结字段相等的行。

mysql> select * from customers;
+----+-------+------+
| id | name  | age  |
+----+-------+------+
|  1 | tom   |   21 |
|  3 | Mike  |   24 |
|  4 | Jack  |   30 |
|  5 | Linda |   25 |
|  6 | Tom   | NULL |
+----+-------+------+

mysql> select * from orders;
+----+----------------+-------+-------------+
| id | order_number   | price | customer_id |
+----+----------------+-------+-------------+
|  1 | Tom_Order001   |   100 |           1 |
|  2 | Tom_Order002   |   200 |           1 |
|  3 | Tom_Order003   |   300 |           1 |
|  4 | Mike_Order001  |   100 |           3 |
|  5 | Jack_Order001  |   200 |           4 |
|  6 | Linda_Order001 |   100 |           5 |
|  7 | UnknownOrder   |   200 |        NULL |
+----+----------------+-------+-------------+

左外连接：
 如果用左外连接的话，指查询出来的是在右表中没有对应左表中的记录以及符合条件（c.id=o.customer_id）的数据，比如：
  customers为左表，orders为右表
  select c.id,o.customer_id,c.name,o.id order_id,order_number from customers c left outer join orders o on c.id=o.customer_id;
  这样的结果显示的是在左表中的某些用户在orders表中没有对应的订单
  
 mysql> select c.id,o.customer_id,c.name,o.id order_id,order_number from customers c left outer join orders o on c.id=o.customer_id;
+----+-------------+-------+----------+----------------+
| id | customer_id | name  | order_id | order_number   |
+----+-------------+-------+----------+----------------+
|  1 |           1 | tom   |        1 | Tom_Order001   |
|  1 |           1 | tom   |        2 | Tom_Order002   |
|  1 |           1 | tom   |        3 | Tom_Order003   |
|  3 |           3 | Mike  |        4 | Mike_Order001  |
|  4 |           4 | Jack  |        5 | Jack_Order001  |
|  5 |           5 | Linda |        6 | Linda_Order001 |
|  6 |        NULL | Tom   |     NULL | NULL           |
+----+-------------+-------+----------+----------------+
7 rows in set (0.00 sec)
  
右外连接：
 正好和左外连接相反，查询出来的结果是在左表中没有对应的项和以及符合条件（c.id=o.customer_id）的数据
 
mysql>  select c.id,o.customer_id,c.name,o.id order_id,order_number from customers c right outer join orders o on c.id=o.customer_id;
+------+-------------+-------+----------+----------------+
| id   | customer_id | name  | order_id | order_number   |
+------+-------------+-------+----------+----------------+
|    1 |           1 | tom   |        1 | Tom_Order001   |
|    1 |           1 | tom   |        2 | Tom_Order002   |
|    1 |           1 | tom   |        3 | Tom_Order003   |
|    3 |           3 | Mike  |        4 | Mike_Order001  |
|    4 |           4 | Jack  |        5 | Jack_Order001  |
|    5 |           5 | Linda |        6 | Linda_Order001 |
| NULL |        NULL | NULL  |        7 | UnknownOrder   |
+------+-------------+-------+----------+----------------+
7 rows in set (0.00 sec)

=============================================================================

mysql> select name,age,sum(price) from customers c left join orders o on c.id=o.customer_id group by c.id;
+-------+------+------------+
| name  | age  | sum(price) |
+-------+------+------------+
| tom   |   21 |        600 |
| Mike  |   24 |        100 |
| Jack  |   30 |        200 |
| Linda |   25 |        100 |
| Tom   | NULL |       NULL |
+-------+------+------------+
5 rows in set (0.00 sec)