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如何在MySQL中使用COUNT执行SELECT？

2024-05-09 11:56:10 504

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)

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如何在MySQL中使用COUNT执行SELECT？

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