在一个 MySQL 查询中计算特定记录(重复)的出现次数
mysqlmysqli database更新于 2024/4/4 6:32:00
为此，使用聚合函数 COUNT() 和 GROUP BY 对这些特定记录进行分组。让我们首先创建一个表 −
mysql> create table DemoTable ( StudentId int NOT NULL AUTO_INCREMENT PRIMARY KEY, StudentSubject varchar(40) ); Query OK, 0 rows affected (5.03 sec)
使用 insert 命令在表中插入一些记录 −
mysql> insert into DemoTable(StudentSubject) values('MySQL');
Query OK, 1 row affected (0.78 sec)
mysql> insert into DemoTable(StudentSubject) values('Java');
Query OK, 1 row affected (0.39 sec)
mysql> insert into DemoTable(StudentSubject) values('MySQL');
Query OK, 1 row affected (1.12 sec)
mysql> insert into DemoTable(StudentSubject) values('MongoDB');
Query OK, 1 row affected (0.24 sec)
mysql> insert into DemoTable(StudentSubject) values('Java');
Query OK, 1 row affected (0.45 sec)
使用 select 语句显示表中的所有记录 −
mysql> select *from DemoTable;
这将产生以下输出 −
+-----------+----------------+ | StudentId | StudentSubject | +-----------+----------------+ | 1 | MySQL | | 2 | Java | | 3 | MySQL | | 4 | MongoDB | | 5 | Java | +-----------+----------------+ 5 rows in set (0.00 sec)
以下是在一个 MySQL 查询中统计特定记录(重复)出现次数的查询 −
mysql> select StudentSubject,count(StudentId) from DemoTable group by StudentSubject;
这将产生以下输出 −
+----------------+------------------+ | StudentSubject | count(StudentId) | +----------------+------------------+ | MySQL | 2 | | Java | 2 | | MongoDB | 1 | +----------------+------------------+ 3 rows in set (0.00 sec)
