例子，mysql如何统计多表交叉组合总数。
复制代码 代码示例:
>mysql -h localhost -u root -p
mysql> use world;
Database changed
mysql> show tables;
+-----------------+
| Tables_in_world |
+-----------------+
| city |
| country |
| countrylanguage |
+-----------------+
3 rows in set (0.00 sec)
mysql> select count(*) from city;
+----------+
| count(*) |
+----------+
| 4079 |
+----------+
1 row in set (0.00 sec)
mysql> select count(*) from country;
+----------+
| count(*) |
+----------+
| 239 |
+----------+
1 row in set (0.00 sec)
mysql> select 4079*239
-> ;
+----------+
| 4079*239 |
+----------+
| 974881 |
+----------+
1 row in set (0.00 sec)
这里就是计算交叉组合数量，因为他们之间一个相同的关联字段
复制代码 代码示例:
mysql> select count(*) from city,country
-> ;
+----------+
| count(*) |
+----------+
| 974881 |
+----------+
1 row in set (0.00 sec)
实际上mysql所做的操作就是把你所查询表的数量相乘得到的结果！
复制代码 代码示例:
mysql> select count(*) from city,country,countrylanguage;
+-----------+
| count(*) |
+-----------+
| 959282904 |
+-----------+
1 row in set (0.02 sec)
mysql> select count(*) from countrylanguage;
+----------+
| count(*) |
+----------+
| 984 |
+----------+
1 row in set (0.00 sec)
mysql> select 4079*239*984;
+--------------+
| 4079*239*984 |
+--------------+
| 959282904 |
+--------------+
1 row in set (0.00 sec)
