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SQL表连接查询

两张表：

 1 mysql> select * from student;
 2 +----+------+------+
 3 | id | name | age  |
 4 +----+------+------+
 5 |  1 | 小王 |   16 |
 6 |  2 | 小红 |   18 |
 7 |  3 | 小明 |   20 |
 8 |  4 | 小李 |   22 |
 9 |  5 | 小强 |   17 |
10 +----+------+------+
11 5 rows in set (0.00 sec)

1 mysql> select * from mark;
2 +----+------+-----------+
3 | id | mark | studentid |
4 +----+------+-----------+
5 |  1 |   61 |         2 |
6 |  2 |   45 |         2 |
7 |  3 |   70 |         3 |
8 +----+------+-----------+
9 3 rows in set (0.00 sec)

一、内连接（inner join)

 1 mysql> select s.name,m.mark from student s,mark m where s.id=m.studentid;
 2 +------+------+
 3 | name | mark |
 4 +------+------+
 5 | 小红 |   61 |
 6 | 小红 |   45 |
 7 | 小明 |   70 |
 8 +------+------+
 9 3 rows in set (0.07 sec)
10 mysql> select s.name,m.mark from student s inner join mark m on s.id=m.studentid;
11 +------+------+
12 | name | mark |
13 +------+------+
14 | 小红 |   61 |
15 | 小红 |   45 |
16 | 小明 |   70 |
17 +------+------+
18 3 rows in set (0.06 sec)

二、左连接（left join)

 1 mysql> select s.name,m.mark from student s left join mark m on s.id=m.studentid;
 2 +------+------+
 3 | name | mark |
 4 +------+------+
 5 | 小红 |   61 |
 6 | 小红 |   45 |
 7 | 小明 |   70 |
 8 | 小王 | NULL |
 9 | 小李 | NULL |
10 | 小强 | NULL |
11 +------+------+
12 6 rows in set (0.07 sec)

三、右连接（right join)

1 mysql> select s.name,m.mark from student s right join mark m on s.id=m.studentid;
2 +------+------+
3 | name | mark |
4 +------+------+
5 | 小红 |   61 |
6 | 小红 |   45 |
7 | 小明 |   70 |
8 +------+------+
9 3 rows in set (0.00 sec)

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原文地址：https://www.cnblogs.com/yexiang520/p/5575787.html