mysql合并统计总数,mysql统计多表交织组合总数
mysql統(tǒng)計多表交叉組合總數(shù)
>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(shù)量，因為他們之間一個相同的關聯(lián)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(shù)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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