如何使用jdbc在sql中计算年级和gpa？

Question

我需要在java中通过GPA搜索学生，并按照附图中的要求使用jbdc连接到数据库。

这些是我在数据库中的表：

 CREATE TABLE Students (
 first_name VARCHAR(20) NOT NULL,
 last_name VARCHAR(20) NOT NULL,
 id VARCHAR(9) NOT NULL,
 PRIMARY KEY (id)
 );



 CREATE TABLE Classes (
 name VARCHAR(50) NOT NULL,
 credits INT NOT NULL,
 PRIMARY KEY (name)
 ); 

 CREATE TABLE IsTaking (
 sid VARCHAR(9) NOT NULL,
 name VARCHAR(50) NOT NULL,
 FOREIGN KEY (sid) REFERENCES Students(id),
 FOREIGN KEY (name) REFERENCES Classes(name)
 );



 CREATE TABLE HasTaken (
 sid VARCHAR(9) NOT NULL,
 name VARCHAR(50) NOT NULL,
 grade CHAR(1) NOT NULL,
 FOREIGN KEY (sid) REFERENCES Students(id),
 FOREIGN KEY (name) REFERENCES Classes(name)
 );

这是我写的java代码：

public static void searchByGPAGreaterThan(Connection conn, Scanner input)  throws SQLException {
    // Construct the SQL query to select the ID, first name, and last name of all students
    // who have taken at least one course and whose GPA is greater than or equal to the threshold.
    System.out.print("Enter GPA threshold: ");
    double threshold = input.nextDouble();
    String query = "SELECT Students.first_name, Students.last_name, Students.id, Classes.credits FROM Students, Classes " +
            "WHERE id IN (SELECT sid FROM HasTaken " + "GROUP BY sid HAVING SUM(CASE grade " +
            "WHEN 'A' THEN 4 " +
            "WHEN 'B' THEN 3 " +
            "WHEN 'C' THEN 2 " +
            "WHEN 'D' THEN 1 " +
            "ELSE 0 END * credits) / SUM(credits) >= ?)";
    try {
        // Create a prepared statement for the query and set the threshold parameter.
        PreparedStatement ps = conn.prepareStatement(query);
        ps.setDouble(1, threshold);
        // Execute the query and get the result set.
        ResultSet rs = ps.executeQuery();
        // Print out the results.
        while (rs.next()) {
            int id = rs.getInt("id");
            String firstName = rs.getString("first_name");
            String lastName = rs.getString("last_name");
            System.out.printf("%d: %s %s\n", id, firstName, lastName);
        }
    } catch (SQLException e) {
        // If there is an error executing the query, print the stack trace.
        e.printStackTrace();
    }
}

此代码输出所有学生，而不是具有指定 gpa 阈值的特定学生。我不知道如何正确计算学生的 gpa，以便输出只打印适当的学生。

我必须用 100 名学生的数据填充数据库，因此我将为一名学生提供样本数据以供参考：

INSERT INTO Classes (name, credits) VALUES
('Calculus I', 4),
('Introduction to Biology', 3),
('World History', 3);

INSERT INTO Students (first_name, last_name, id) VALUES
('Amabelle', 'Jimmes', '10000');

INSERT INTO IsTaking (sid, name) VALUES
(10000, 'Introduction to Biology'),
(10000, 'Calculus I'),
(10000, 'World History');

INSERT INTO HasTaken (sid, name, grade) VALUES
(10000, 'Introduction to Biology', 'A'),
(10000, 'Calculus I', 'B'),
(10000, 'World History', 'C');

Answer 1

我会更改查询，以便它将原始数据（每个学生）返回给 java 类，然后进行必要的计算

select s.*, group_concat(case ht.grade when 'A' then 4 when 'B' then 3 when 'C' then 2 when 'D' then 1 end) grade from Students s
join HasTaken ht on ht.sid = s.id
join Classes c on c.name = ht.name
group by s.id
;

会产生

first_name  last_name   id  grade
Amabelle    Jimmes  10000   4,3,2