联合查询也称为多表查询，是将多个表联合到一起进行查询；

笛卡尔积是联合查询的基础，笛卡尔积其实就是一种排列组合，把两张表的记录尽可能地排列组合出n种情况：

以两张表：班级表与学生表为例，计算这两个表的笛卡尔积：

笛卡尔积就是得到了一张更大的表，其列数为两个表列数之和，行数为两个表列数之积；

试在testdemo1数据库下创建以下表：

mysql> show tables;
+---------------------+
| Tables_in_testdemo1 |
+---------------------+
| classes             |
| course              |
| score               |
| student             |
+---------------------+
4 rows in set (0.00 sec)

 表的结构与内容分别为：

（1）student表：

mysql> desc student;
+------------+-------------+------+-----+---------+----------------+
| Field      | Type        | Null | Key | Default | Extra          |
+------------+-------------+------+-----+---------+----------------+
| id         | int(11)     | NO   | PRI | NULL    | auto_increment |
| sn         | varchar(20) | YES  |     | NULL    |                |
| name       | varchar(20) | YES  |     | NULL    |                |
| qq_mail    | varchar(20) | YES  |     | NULL    |                |
| classes_id | int(11)     | YES  |     | NULL    |                |
+------------+-------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)mysql> select* from student;
+----+-------+------------+------------------+------------+
| id | sn    | name       | qq_mail          | classes_id |
+----+-------+------------+------------------+------------+
|  1 | 09982 | 黑旋风李逵 | xuanfeng@qq.com  |          1 |
|  2 | 00835 | 菩提老祖   | NULL             |          1 |
|  3 | 00391 | 白素贞     | NULL             |          1 |
|  4 | 00031 | 许仙       | xuxian@qq.com    |          1 |
|  5 | 00054 | 不想毕业   | NULL             |          1 |
|  6 | 51234 | 好好说话   | say@qq.com       |          2 |
|  7 | 83223 | tellme     | NULL             |          2 |
|  8 | 09527 | 老外学中文 | foreigner@qq.com |          2 |
+----+-------+------------+------------------+------------+
8 rows in set (0.00 sec)

（2）classes表：

mysql> desc classes;
+-------+--------------+------+-----+---------+----------------+
| Field | Type         | Null | Key | Default | Extra          |
+-------+--------------+------+-----+---------+----------------+
| id    | int(11)      | NO   | PRI | NULL    | auto_increment |
| name  | varchar(20)  | YES  |     | NULL    |                |
| desc  | varchar(100) | YES  |     | NULL    |                |
+-------+--------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)mysql> select* from classes;
+----+-------------------+-----------------------------------------------+
| id | name              | desc                                          |
+----+-------------------+-----------------------------------------------+
|  1 | 计算机系2019级1班 | 学习了计算机原理、C和Java语言、数据结构和算法 |
|  2 | 中文系2019级3班   | 学习了中国传统文学                            |
|  3 | 自动化2019级5班   | 学习了机械自动化                              |
+----+-------------------+-----------------------------------------------+
3 rows in set (0.00 sec)

（3）course表：

mysql> desc course;
+-------+-------------+------+-----+---------+----------------+
| Field | Type        | Null | Key | Default | Extra          |
+-------+-------------+------+-----+---------+----------------+
| id    | int(11)     | NO   | PRI | NULL    | auto_increment |
| name  | varchar(20) | YES  |     | NULL    |                |
+-------+-------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)mysql> select* from course;
+----+--------------+
| id | name         |
+----+--------------+
|  1 | Java         |
|  2 | 中国传统文化 |
|  3 | 计算机原理   |
|  4 | 语文         |
|  5 | 高阶数学     |
|  6 | 英文         |
+----+--------------+
6 rows in set (0.00 sec)

（4）score表： 

mysql> desc score;
+------------+--------------+------+-----+---------+-------+
| Field      | Type         | Null | Key | Default | Extra |
+------------+--------------+------+-----+---------+-------+
| score      | decimal(3,1) | YES  |     | NULL    |       |
| student_id | int(11)      | YES  |     | NULL    |       |
| course_id  | int(11)      | YES  |     | NULL    |       |
+------------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)mysql> select* from score;
+-------+------------+-----------+
| score | student_id | course_id |
+-------+------------+-----------+
|  70.5 |          1 |         1 |
|  98.5 |          1 |         3 |
|  33.0 |          1 |         5 |
|  98.0 |          1 |         6 |
|  60.0 |          2 |         1 |
|  59.5 |          2 |         5 |
|  33.0 |          3 |         1 |
|  68.0 |          3 |         3 |
|  99.0 |          3 |         5 |
|  67.0 |          4 |         1 |
|  23.0 |          4 |         3 |
|  56.0 |          4 |         5 |
|  72.0 |          4 |         6 |
|  81.0 |          5 |         1 |
|  37.0 |          5 |         5 |
|  56.0 |          6 |         2 |
|  43.0 |          6 |         4 |
|  79.0 |          6 |         6 |
|  80.0 |          7 |         2 |
|  92.0 |          7 |         6 |
+-------+------------+-----------+
20 rows in set (0.00 sec)

在该数据库中四张表，三个实体：学生、班级、课程；

其中学生和班级是一对多关系，学生和课程是多对多关系（成绩表是关联表），班级和课程之间没有直接的关联关系；