Question

我有一些代码，我希望能够在允许学生注册之前运行以检查课程是否已满。我使用String currentNumberInClass创建了classFullCheck（）函数，该函数从sql表中获取计数并将其与另一个表中的class_size进行比较。我想也许我正在返回class_id而不是count（*）号，所以这不能正常工作。我也不确定将classFullCheck（）函数插入addClass（）函数的位置，以使此检查正常工作。

任何建议都将不胜感激！

public static void classFullCheck() {
        try {
            String currentNumberInClass = ("SELECT class_id, COUNT(*) FROM ClassSelector.student_x_class WHERE class_id = " + selectedClass);
            rs = myStmt.executeQuery(currentNumberInClass);
            String classSize = ("SELECT class_size FROM ClassSelector.classes WHERE class_id = " + selectedClass);
            rs = myStmt.executeQuery(classSize);
                if (currentNumberInClass.compareTo(classSize) >= 0){
                    System.out.println("Sorry, this class is Full!");
            }
        } catch (java.sql.SQLException SQL) {
            SQL.printStackTrace();
        }
    }

    public static void addClass() {

        try {
            rs = myStmt.executeQuery("SELECT * FROM ClassSelector.classes");
            while (rs.next()) {
                String availableClasses = rs.getString("class_id") + "\t" + rs.getString("class_name") + "\t" + rs.getString("description");
                System.out.println(availableClasses);
            }
            System.out.println("Enter Class ID from Classes Listed Above to Join: ");
            selectedClass = sc.nextLine();
            rs = myStmt.executeQuery("SELECT * FROM ClassSelector.classes WHERE class_id = " + selectedClass);
            while (rs.next()) {
                String innerJoin = (userEnterIdAsName + " has been added to " + rs.getString("class_name") + " " + rs.getString("class_id"));
                System.out.println(innerJoin);
                String student_x_classJoin = "INSERT INTO student_x_class" + "(student_id, student_name, class_id, class_name)" + "VALUES (?, ?, ?, ?)";
                PreparedStatement pStmt = con.prepareStatement(student_x_classJoin);
                    pStmt.setString(1, user_entered_student_id);
                    pStmt.setString(2, userEnterIdAsName);
                    pStmt.setString(3, rs.getString("class_id"));
                    pStmt.setString(4, rs.getString("class_name"));
                    pStmt.executeUpdate();
                    System.out.println("Would you like to enroll " + userEnterIdAsName + " into another class? (Y/N)");
                    String addAdditionalClass = sc.nextLine();
                    if (addAdditionalClass.equalsIgnoreCase("Y")) {
                        addClass();
                    } else if (addAdditionalClass.equalsIgnoreCase("N")) {
                        return;
                    }
                }
        }
        catch (java.sql.SQLException SQL) {
            System.out.println("Wait, This Student is already enrolled in this class!");
        }
    }

Answer 1

你不需要在classFullCheck（）中做两个Query 您可以通过以下查询执行此操作：

"SELECT claclass, class_size, (SELECT COUNT(1) 
FROM ClassSelector.student_x_class WHERE class_id = c.class_id ) as current_student_count
FROM ClassSelector.classes c WHERE class_id = " + selectedClass

当current_student_count＆lt; class_size。但更好的解决方案是打印列表类，其中包含class_size＆gt;的学生数量。 current_student_count做一个选择。

Answer 2

因此在java jdbc中，您可以通过以下方式从sql中检索返回的结果。

public boolean isClassFull(int classId) {
    // Sql to get count
    String classCountSql = "SELECT Count(*) as classCount FROM ClassSelector.student_x_class WHERE class_id = ?;";
    // Sql to get max count for a class
    String classSizeSql = "SELECT class_size as classSize FROM ClassSelector.classes WHERE class_id = ?;";
    int classCount = 0;
    int classSize = 0;
    try(Connection connection = databaseConnection.getConnection()) {
        try(PreparedStatement preparedStatement = connection.prepareStatement(classCountSql)) {
            preparedStatement.setInt(1, classId);

            try(ResultSet resultSet = preparedStatement.executeQuery()) {
                if(resultSet.next()){
                    classCount = resultSet.getInt("classCount");
                }
            }

        }
    try(PreparedStatement preparedStatement = connection.prepareStatement(classSizeSql)) {
            preparedStatement.setInt(1, classId);

            try(ResultSet resultSet = preparedStatement.executeQuery()) {
                if(resultSet.next()){
                    classCount = resultSet.getInt("classSize");
                }
            }
        }
    } catch(SQLException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
    return classCount >= classSize;
}

使用Java在SQL中返回count（*）值

2 个答案: