mysql統計多表交叉組合總數

>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)

這里就是計算交叉組合數量，因為他們之間一個相同的關聯字段

mysql> select count(*) from city,country

-> ;

+----------+

| count(*) |

+----------+

| 974881 |

+----------+

1 row in set (0.00 sec)

實際上mysql所做的操作就是把你所查詢表的數量相乘得到的結果！

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)

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