如何在使用java創建新帳戶之前檢查sql數據庫中是否已存在用戶名

Question

當我想在用戶創建新帳戶或更新他/她的用戶名時提示消息並且sql數據庫中已存在該用戶名時，我如何輸入代碼。 當我在CreateNewUserAccount類上單擊“保存”按鈕時，這是我的代碼的一部分：

   String sql = "insert into tblUserInfo (UserID, LastName, FirstName, PositionID, Username, Password) values(?, ?, ?, ?, ?, ?) " ;
    try{
        pst = conn.prepareStatement(sql);
        pst.setString(1, txtUserID.getText());
        pst.setString(2, txtLastName.getText());
        pst.setString(3, txtFirstName.getText());
        pst.setString(4, (String) comboPosition.getSelectedItem().toString().substring(0, 1));
        pst.setString(5, txtUsername.getText());
        pst.setString(6, txtPassword.getText());
        pst.execute();
        if(txtConfirmPassword.getText().equals(txtPassword.getText())){
        JOptionPane.showMessageDialog(null, "Saved New User Account");
        clear();
        }
        else{
          JOptionPane.showMessageDialog(null, "Incompatible Password");
          txtConfirmPassword.setText("");
        }
        //UpdateEmployeeTable();
    }catch(Exception e){
        JOptionPane.showMessageDialog(null, e);
    }

以及我的UserAccount類，用戶可以在其中更新/編輯他們的信息，包括用戶名：

  String sql = "update tblUserInfo set LastName = ?, FirstName = ?, PositionID = ?, Username = ?, Password = ? where UserID= ?";

 try {
    pst = conn.prepareStatement(sql);

    pst.setString(1, txtLastName.getText());
    pst.setString(2, txtFirstName.getText());
    pst.setString(3, (String) comboPos.getSelectedItem().toString().substring(0, 1));
    pst.setString(4, txtUserName.getText());
    pst.setString(5, txtPassword.getText());
    pst.setString(6, txtUserID.getText());
    pst.executeUpdate();
    JOptionPane.showMessageDialog(null, "User Account Updated");

} catch (Exception e) {
    e.printStackTrace();
    JOptionPane.showMessageDialog(null, e);
}

我怎么能對此進行檢查？ 提前致謝

Answer 1

我會用這樣的東西

IF EXISTS (SELECT * FROM Table1 WHERE Column1='SomeValue')
    UPDATE Table1 SET (...) WHERE Column1='SomeValue'
ELSE
    INSERT INTO Table1 VALUES (...)

http://blogs.msdn.com/b/miah/archive/2008/02/17/sql-if-exists-update-else-insert.aspx

Answer 2

一種選擇是對用戶名不相同的用戶名進行計數查詢，例如：

select count(Username) from tblUserInfo where UserId != ?

如果您發現結果大於零，則該用戶ID由其他人使用，則顯示警告。

Answer 3

你可以這樣做

Statement st=con.getConnection(.....);
String query= "select username from tables where username="uservalue"";
 ResultSet rs=st.executeQuery(query);
int value=0;
 while(rs.next())
 {
  value++;
  }
  if(value>1)
  {
  String query= "select username from tables where username="uservalue"";
   ResultSet rs=st.executeQuery(query);
       while(rs.next())
      {
        String user=rs.getString("username");

       }
          String updatequery="update tables set username="updatevalue"   where username="updatevalue"";
       int q=st.executeupdate(updatequery);
     }
       else 
     {
         insert query..........
   }