mysql统计多表交叉组合总数实例

>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) 

mysql> select count(*) from city,country 
    -> ; 
+----------+ 
| count(*) | 
+----------+ 
|   974881 | 
+----------+ 
1 row in set (0.00 sec) 

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)