Java 如果数据库中不存在ID,则返回消息对话框
如果用户查询我的SQLite数据库中不存在的ID,我很难让我的前端返回一个JOptionPane,说明该ID不存在,如果该ID存在,则显示该ID的记录。我尝试了isEmpty(),这显然是错误的 我的代码:Java 如果数据库中不存在ID,则返回消息对话框,java,sqlite,select,null,Java,Sqlite,Select,Null,如果用户查询我的SQLite数据库中不存在的ID,我很难让我的前端返回一个JOptionPane,说明该ID不存在,如果该ID存在,则显示该ID的记录。我尝试了isEmpty(),这显然是错误的 我的代码: String SelectRecords = "SELECT * FROM Student WHERE ID_Num = ?"; String ID_Number = id_input.getText(); PreparedStatement
String SelectRecords = "SELECT * FROM Student WHERE ID_Num = ?";
String ID_Number = id_input.getText();
PreparedStatement ps = c.prepareStatement(SelectRecords);
ps.setString(1, ID_Number);
ResultSet rs = ps.executeQuery();
while(rs.next()){
String IDNum = rs.getString("ID_Num");
String LastName = rs.getString("Stud_Name_Last");
String FirstName = rs.getString("Stud_Name_First");
String DaysPresent = rs.getString("days_present");
String DaysAbsent = rs.getString("days_absent");
String DropStatus = rs.getString("drop_status");
String Course = rs.getString("course");
String LastAttended = rs.getString("last_attended_class");
String StudentRecords = "ID Number: "+IDNum+"\n"+"Last Name"+LastName+"\n"+"First Name: "+FirstName+"\n"+"Days Present: "+DaysPresent+"\n"+"Days Absent: "+DaysAbsent+"\n"+"Drop Status: "+DropStatus+"\n"+"Course: "+Course+"\n"+"Last Attended On: "+LastAttended;
Output_Label.setText(LastName+", "+FirstName+"(Retrieving...)");
if(rs.next() == true){
JOptionPane.showMessageDialog(null, StudentRecords, LastName+", "+FirstName, JOptionPane.INFORMATION_MESSAGE);
}
else{
JOptionPane.showMessageDialog(null, "Does not exist");
}
您正在调用两次
rs.next()
,这只有在至少有两条记录时才有意义。
假设只有一个学生具有特定的ID,while
循环也没有意义
程序的逻辑可能如下所示:
ResultSet rs = ps.executeQuery();
if (rs.next()) {
// found
} else {
// not found
}
if(IDNum==null)JOptionPane.showMessageDialog(null,“不存在”)代码>@juergend对话框在id存在但对不存在的id的查询没有响应时弹出。在IDE中设置断点,在运行时查看idnum
的值,您将看到发生了什么。