mysql> desc student;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| ID | int(11) | NO | PRI | 0 | |
| NAME | varchar(16) | YES | | NULL | |
| AGE | int(11) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
3 rows in set

mysql> desc sc;
+-------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+----------------+
| ID | int(11) | NO | PRI | NULL | auto_increment |
| SID | int(11) | YES | | NULL | |
| CID | int(11) | YES | MUL | NULL | |
| SCORE | int(11) | YES | | NULL | |
+-------+---------+------+-----+---------+----------------+
4 rows in set

mysql> select * from student;
+-------+----------+-----+
| ID | NAME | AGE |
+-------+----------+-----+
| 10001 | Andy | 26 |
| 10002 | Bill | 27 |
| 10003 | Caroline | 34 |
| 10004 | David | 46 |
+-------+----------+-----+
4 rows in set

mysql> select * from sc;
+----+-------+-----+-------+
| ID | SID | CID | SCORE |
+----+-------+-----+-------+
| 1 | 10001 | 101 | 78 |
| 2 | 10001 | 102 | 67 |
| 3 | 10008 | 103 | 100 |
+----+-------+-----+-------+
3 rows in set
-----------------------------------------------------------------------------------------------------------------
内连接
mysql> select student.*, sc.* from student inner join sc on student.id = sc.sid;
+-------+------+-----+----+-------+-----+-------+
| ID | NAME | AGE | ID | SID | CID | SCORE |
+-------+------+-----+----+-------+-----+-------+
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |
+-------+------+-----+----+-------+-----+-------+
2 rows in set

内连接等价于我们平时的自然连接，也就是：
mysql> select student.*, sc.* from student,sc where student.id = sc.sid;
+-------+------+-----+----+-------+-----+-------+
| ID | NAME | AGE | ID | SID | CID | SCORE |
+-------+------+-----+----+-------+-----+-------+
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |
+-------+------+-----+----+-------+-----+-------+
2 rows in set

-----------------------------------------------------------------------------------------------------------------
左连接
考虑下面的需求，我想列出所有学生对应的成绩，一个学生可能多个成绩，也可能没有成绩。有多个成绩把多个成绩列出来，没有成绩的话，成绩这些字段的值使用NULL填充。怎么解决这个问题？
使用左连接，student表 left join sc表，如下：
mysql> select student.*, sc.* from student left join sc on student.id = sc.sid;
+-------+----------+-----+------+-------+------+-------+
| ID | NAME | AGE | ID | SID | CID | SCORE |
+-------+----------+-----+------+-------+------+-------+
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |
| 10002 | Bill | 27 | NULL | NULL | NULL | NULL |
| 10003 | Caroline | 34 | NULL | NULL | NULL | NULL |
| 10004 | David | 46 | NULL | NULL | NULL | NULL |
+-------+----------+-----+------+-------+------+-------+
5 rows in set

-----------------------------------------------------------------------------------------------------------------
右连接
考虑下面的需求，我想列出所有成绩对应的学生，一个成绩有对应的学生，也可能没有对应的学生，比如这个学生开除了。有学生就把学生列出来，没有学生的话，学生这些字段的值使用NULL填充。怎么解决这个问题？
使用右连接，student表 right join sc表，如下：
mysql> select student.*, sc.* from student right join sc on student.id = sc.sid;
+-------+------+------+----+-------+-----+-------+
| ID | NAME | AGE | ID | SID | CID | SCORE |
+-------+------+------+----+-------+-----+-------+
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |
| NULL | NULL | NULL | 3 | 10008 | 103 | 100 |
+-------+------+------+----+-------+-----+-------+
3 rows in set

根据对称性，A left join B 等价于 B right join A
-----------------------------------------------------------------------------------------------------------------
外连接
左连接也叫左外连接（同理右连接），这里的外连接也叫全外连接。
考虑下面的需求，我想列出所有学生对应的所有成绩，这里存在学生可能没有成绩，成绩也可能没有对应的学生，没有的话，也要列出来，这些字段的值使用NULL填充。
目前，mysql不支持外连接，解决办法是使用union组合查询，把左连接和右连接的结果合并。如下：
mysql> select student.*, sc.* from student left join sc on student.id = sc.sid union select student.*, sc.* from student right join sc on student.id = sc.sid;
+-------+----------+------+------+-------+------+-------+
| ID | NAME | AGE | ID | SID | CID | SCORE |
+-------+----------+------+------+-------+------+-------+
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |
| 10002 | Bill | 27 | NULL | NULL | NULL | NULL |
| 10003 | Caroline | 34 | NULL | NULL | NULL | NULL |
| 10004 | David | 46 | NULL | NULL | NULL | NULL |
| NULL | NULL | NULL | 3 | 10008 | 103 | 100 |
+-------+----------+------+------+-------+------+-------+
6 rows in set

注意：这里的union自动去除了重复行，如果不想去除重复行，使用union all

-----------------------------------------------------------------------------------------------------------------
还有一点需要注意：就是on 之后的条件，如下：
mysql> select student.*, sc.* from student left join sc on student.id = sc.sid;
+-------+----------+-----+------+-------+------+-------+
| ID | NAME | AGE | ID | SID | CID | SCORE |
+-------+----------+-----+------+-------+------+-------+
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |
| 10002 | Bill | 27 | NULL | NULL | NULL | NULL |
| 10003 | Caroline | 34 | NULL | NULL | NULL | NULL |
| 10004 | David | 46 | NULL | NULL | NULL | NULL |
+-------+----------+-----+------+-------+------+-------+
5 rows in set

mysql> select student.*, sc.* from student left join sc on student.id = sc.sid and sc.cid=101;
+-------+----------+-----+------+-------+------+-------+
| ID | NAME | AGE | ID | SID | CID | SCORE |
+-------+----------+-----+------+-------+------+-------+
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |
| 10002 | Bill | 27 | NULL | NULL | NULL | NULL |
| 10003 | Caroline | 34 | NULL | NULL | NULL | NULL |
| 10004 | David | 46 | NULL | NULL | NULL | NULL |
+-------+----------+-----+------+-------+------+-------+
4 rows in set

mysql> select student.*, sc.* from student left join sc on student.id = sc.sid where sc.cid=101;
+-------+------+-----+----+-------+-----+-------+
| ID | NAME | AGE | ID | SID | CID | SCORE |
+-------+------+-----+----+-------+-----+-------+
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |
+-------+------+-----+----+-------+-----+-------+
1 row in set

这里看出第二个查询和第三个查询的区别，换一种写法就很清楚了。
mysql> select student.*, sc.* from student left join sc on (student.id = sc.sid and sc.cid=101);
+-------+----------+-----+------+-------+------+-------+
| ID | NAME | AGE | ID | SID | CID | SCORE |
+-------+----------+-----+------+-------+------+-------+
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |
| 10002 | Bill | 27 | NULL | NULL | NULL | NULL |
| 10003 | Caroline | 34 | NULL | NULL | NULL | NULL |
| 10004 | David | 46 | NULL | NULL | NULL | NULL |
+-------+----------+-----+------+-------+------+-------+
4 rows in set

mysql> select student.*, sc.* from student left join sc on (student.id = sc.sid) where sc.cid=101;
+-------+------+-----+----+-------+-----+-------+
| ID | NAME | AGE | ID | SID | CID | SCORE |
+-------+------+-----+----+-------+-----+-------+
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |
+-------+------+-----+----+-------+-----+-------+
1 row in set

mysql> select student.*, sc.* from student left join sc on (student.id = sc.sid where sc.cid=101);
1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'where sc.cid=101)' at line 1

-----------------------------------------------------------------------------------------------------------------
select student.*, sc.* from student left join sc on (student.id = sc.sid and sc.cid=101); 相当于：
1、内部连接
mysql> select student.*, sc.* from student inner join sc on student.id = sc.sid;
+-------+------+-----+----+-------+-----+-------+
| ID | NAME | AGE | ID | SID | CID | SCORE |
+-------+------+-----+----+-------+-----+-------+
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |
+-------+------+-----+----+-------+-----+-------+
2 rows in set
2、选择出sc.cid=101，再进行左连接，没有成绩的使用NULL填充

-----------------------------------------------------------------------------------------------------------------
select student.*, sc.* from student left join sc on (student.id = sc.sid) where sc.cid=101; 相当于：
1、左连接
mysql> select student.*, sc.* from student left join sc on student.id = sc.sid;
+-------+----------+-----+------+-------+------+-------+
| ID | NAME | AGE | ID | SID | CID | SCORE |
+-------+----------+-----+------+-------+------+-------+
| 10001 | Andy | 26 | 1 | 10001 | 101 | 78 |
| 10001 | Andy | 26 | 2 | 10001 | 102 | 67 |
| 10002 | Bill | 27 | NULL | NULL | NULL | NULL |
| 10003 | Caroline | 34 | NULL | NULL | NULL | NULL |
| 10004 | David | 46 | NULL | NULL | NULL | NULL |
+-------+----------+-----+------+-------+------+-------+
5 rows in set
2、再选出 sc.cid=101