如何在MySQL中使用COUNT执行SELECT？
要使用COUNT执行SELECT，请使用聚合函数COUNT()。让我们首先创建一个表-
mysql> create table DemoTable ( Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, Name varchar(100), Subject varchar(100) );
使用插入命令在表中插入一些记录-
mysql> insert into DemoTable(Name,Subject) values('John','MySQL');
mysql> insert into DemoTable(Name,Subject) values('John','Java');
mysql> insert into DemoTable(Name,Subject) values('Carol','MongoDB');
mysql> insert into DemoTable(Name,Subject) values('Carol','Java');
mysql> insert into DemoTable(Name,Subject) values('Carol','MySQL');
mysql> insert into DemoTable(Name,Subject) values('John','MySQL');
mysql> insert into DemoTable(Name,Subject) values('Carol','MongoDB');使用select语句显示表中的所有记录-
mysql> select *from DemoTable;
输出结果
+----+-------+---------+ | Id | Name | Subject | +----+-------+---------+ | 1 | John | MySQL | | 2 | John | Java | | 3 | Carol | MongoDB | | 4 | Carol | Java | | 5 | Carol | MySQL | | 6 | John | MySQL | | 7 | Carol | MongoDB | +----+-------+---------+ 7 rows in set (0.00 sec)
以下是在MySQL中使用count选择的查询-
mysql> select Name,Subject,count(Subject) from DemoTable group by Subject,Name;
输出结果
+-------+---------+----------------+ | Name | Subject | count(Subject) | +-------+---------+----------------+ | John | MySQL | 2 | | John | Java | 1 | | Carol | MongoDB | 2 | | Carol | Java | 1 | | Carol | MySQL | 1 | +-------+---------+----------------+ 5 rows in set (0.00 sec)