Question

我很难在学生的专栏中显示课程数量。

我有以下查询，但我不确定COUNT究竟是在计算什么，以及如何显示每个学生的计数：

CREATE VIEW Busy_Student AS
SELECT First_Name, Last_Name, Student.Student_ID
FROM Student 
INNER JOIN Enrollment ON Enrollment.Student_ID= Student.Student_ID
GROUP BY Student.Student_ID, Student.First_Name, Student.Last_Name 
HAVING COUNT(*) > 2

为所有注册超过2个班级的学生创建一个视图调用BUSY_STUDENT，用于存储连接的名称（名字和姓氏），学生ID和注册课程数。将列命名为FULL_NAME，STUDENT_ID和ENROLL_NUM。

Answer 1

您的SELECT语句应该是这样的：

SELECT First_Name + ' ' + Last_Name AS [FULL_NAME],
Student.Student_ID, COUNT(*) AS [ENROL_NUM]
FROM Student 
INNER JOIN Enrollment ON Enrollment.Student_ID= Student.Student_ID
GROUP BY Student.Student_ID, Student.First_Name, Student.Last_Name 
HAVING COUNT(*) > 2

Answer 2

你非常接近它：

CREATE VIEW Busy_Student AS
SELECT
    CONCAT(First_Name, ' ', Last_Name) AS FULL_NAME,
    Student.Student_ID AS STUDENT_ID,
    COUNT(Enrollment.Enrollment_id) AS ENROLL_NUM
FROM Student 
INNER JOIN Enrollment USING(Student_ID)
GROUP BY Student.Student_ID
HAVING ENROLL_NUM > 2

计数是根据与学生ID匹配的Enrollment行数完成的。由于您只给出了内连接列，因此我假设每个注册都由列Enrollment_id唯一标识。您应该根据您的架构调整该列名称。

使用

First_Name + ' ' + Last_Name AS FULL_NAME

用于SQL Server。

SQL学生数据库

2 个答案: