1.查询student表
mysql> select * from student;
+--------+------+------+
| stu_id | name | c_id |
+--------+------+------+
| 001 | jim | 1 |
| 002 | tom | 2 |
+--------+------+------+
2 rows in set (0.00 sec)2.查询class表
mysql> select * from grade;
+------+--------+
| g_id | gname |
+------+--------+
| 1 | grade1 |
| 2 | grade2 |
+------+--------+
2 rows in set (0.00 sec)
3.查询grade表
mysql> select * from class;
+------+--------+------+
| c_id | cname | g_id |
+------+--------+------+
| 1 | class1 | 1 |
| 2 | class2 | 2 |
+------+--------+------+
2 rows in set (0.00 sec)4.笛卡尔积
mysql> select * from student,class,grade;
+--------+------+------+------+--------+------+------+--------+
| stu_id | name | c_id | c_id | cname | g_id | g_id | gname |
+--------+------+------+------+--------+------+------+--------+
| 001 | jim | 1 | 1 | class1 | 1 | 1 | grade1 |
| 002 | tom | 2 | 1 | class1 | 1 | 1 | grade1 |
| 001 | jim | 1 | 2 | class2 | 2 | 1 | grade1 |
| 002 | tom | 2 | 2 | class2 | 2 | 1 | grade1 |
| 001 | jim | 1 | 1 | class1 | 1 | 2 | grade2 |
| 002 | tom | 2 | 1 | class1 | 1 | 2 | grade2 |
| 001 | jim | 1 | 2 | class2 | 2 | 2 | grade2 |
| 002 | tom | 2 | 2 | class2 | 2 | 2 | grade2 |
+--------+------+------+------+--------+------+------+--------+
8 rows in set (0.00 sec)5.先用笛卡尔积查所有数据 再筛选正确数据
mysql> select * from student s,class c,grade g where s.c_id=c.c_id and c.g_id =
g.g_id;
+--------+------+------+------+--------+------+------+--------+
| stu_id | name | c_id | c_id | cname | g_id | g_id | gname |
+--------+------+------+------+--------+------+------+--------+
| 001 | jim | 1 | 1 | class1 | 1 | 1 | grade1 |
| 002 | tom | 2 | 2 | class2 | 2 | 2 | grade2 |
+--------+------+------+------+--------+------+------+--------+
2 rows in set (0.00 sec)