如何在MySQL中使用COUNT执行SELECT呢？

要使用COUNT()聚合函数执行SELECT，请先创建一个表 –

mysql> create table DemoTable 
    (
    Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
    Name varchar(100),
    Subject varchar(100)
    );
Query OK, 0 rows affected (0.54 sec)

使用insert语句在表中插入一些记录 –

mysql> insert into DemoTable(Name,Subject) values('John','MySQL');
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable(Name,Subject) values('John','Java');
Query OK, 1 row affected (0.17 sec)
mysql> insert into DemoTable(Name,Subject) values('Carol','MongoDB');
Query OK, 1 row affected (0.15 sec)
mysql> insert into DemoTable(Name,Subject) values('Carol','Java');
Query OK, 1 row affected (0.11 sec)
mysql> insert into DemoTable(Name,Subject) values('Carol','MySQL');
Query OK, 1 row affected (0.16 sec)
mysql> insert into DemoTable(Name,Subject) values('John','MySQL');
Query OK, 1 row affected (0.23 sec)
mysql> insert into DemoTable(Name,Subject) values('Carol','MongoDB');
Query OK, 1 row affected (0.26 sec)

使用select语句显示表中的所有记录 –

mysql> select *from DemoTable;

阅读更多：MySQL 教程

输出

+----+-------+---------+
| 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行(0.00秒) 

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

+-------+---------+----------------+
| Name  | Subject | count(Subject) |
+-------+---------+----------------+
| John  | MySQL   | 2              |
| John  | Java    | 1              |
| Carol | MongoDB | 2              |
| Carol | Java    | 1              |
| Carol | MySQL   | 1              |
+-------+---------+----------------+
5行(0.00秒)