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使用条件对值进行计数并在MySQL的不同列中显示结果？

2024-04-28 04:47:06 424

让我们首先创建一个表-

mysql> create table DemoTable1485
    -> (
    -> StudentId int NOT NULL AUTO_INCREMENT PRIMARY KEY,
    -> StudentName varchar(20),
    -> StudentSubject varchar(20)
    -> );

使用插入命令在表中插入一些记录-

mysql> insert into DemoTable1485(StudentName,StudentSubject) values('Chris','MySQL');
mysql> insert into DemoTable1485(StudentName,StudentSubject) values('Robert','MongoDB');
mysql> insert into DemoTable1485(StudentName,StudentSubject) values('Robert','MongoDB');
mysql> insert into DemoTable1485(StudentName,StudentSubject) values('Chris','Java');

使用select语句显示表中的所有记录-

mysql> select * from DemoTable1485;

这将产生以下输出-

+-----------+-------------+----------------+
| StudentId | StudentName | StudentSubject |
+-----------+-------------+----------------+
|         1 | Chris       | MySQL          |
|         2 | Robert      | MongoDB        |
|         3 | Robert      | MongoDB        |
|         4 | Chris       | Java           |
+-----------+-------------+----------------+
4 rows in set (0.00 sec)

这是根据条件对值进行计数的查询-

mysql> select StudentSubject,
    -> sum(case when StudentName = 'Chris' THEN 1 ELSE 0 END) Chris_Count,
    -> sum(case when StudentName = 'Robert' THEN 1 ELSE 0 END) Robert_Count
    -> from DemoTable1485
    -> group by StudentSubject;

这将产生以下输出-

+----------------+-------------+--------------+
| StudentSubject | Chris_Count | Robert_Count |
+----------------+-------------+--------------+
| MySQL          |           1 |            0 |
| MongoDB        |           0 |            2 |
| Java           |           1 |            0 |
+----------------+-------------+--------------+
3 rows in set (0.00 sec)

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使用条件对值进行计数并在MySQL的不同列中显示结果？

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