c# - 将空值和其他值从 sql 数据库检索到文本框

Question

我在从 Visual Studio 的 SQL 数据库中检索值时遇到问题。我在 GUEST 表中有列（GuestName、GuestPassportNo、GuestCountry、RoomID、HotelPackageID），其中 HotelPackageID 包含 NULL 值，而其余的则带有值。当 HotelPackageID 为 NULL 时，我总是被抛出 else 语句（未找到记录）。请问有人有解决办法吗？这是我的代码：

SqlConnection myConnect = new SqlConnection(strConnectionString);

string strCommandText = "SELECT Guest.GuestPassportNo, 
                                Guest.GuestName,   
                                Guest.GuestCountry, 
                                Guest.RoomID, 
                                Room.RoomDescription, 
                                Room.RoomPrice, 
                                HotelPackages.PackageName, 
                                HotelPackages.PackagePrice ";

strCommandText += " FROM Guest, Room, HotelPackages";

strCommandText +=" WHERE Guest.RoomID=Room.RoomID 
                   AND Guest.GuestPassportNo=@guestpno 
                   AND HotelPackages.PackageID=Guest.HotelPackageID 
                   AND Guest.GuestCountry=@guestcountry;

SqlCommand cmd = new SqlCommand(strCommandText, myConnect);
cmd.Parameters.AddWithValue("@guestpno", txtPassportNo.Text);
cmd.Parameters.AddWithValue("@guestcountry", txtGuestCountry.Text);

try
{
myConnect.Open();
SqlDataReader reader = cmd.ExecuteReader();

if(reader.Read())
{
txtGuestName.Text = reader["GuestName"].ToString();
txtRoomDescription.Text = reader["RoomDescription"].ToString();
txtRoomPrice.Text = reader["RoomPrice"].ToString();
txtHotelPackage.Text = reader["PackageName"].ToString();
txtPackagePrice.Text = reader["PackagePrice"].ToString();
MessageBox.Show("Guest Record Found!");
}

else
{
Messagebox.Show("No record Found");
}

reader.Close();
}

catch(Exception ex)
{
MessageBox.Show(ex.message);
}

finally
{
myConnect.Close();
}

Answer 1

您需要更改您的查询：

strCommandText +=" WHERE Guest.RoomID=Room.RoomID AND Guest.GuestPassportNo=@guestpno AND HotelPackages.PackageID=Guest.HotelPackageID AND Guest.GuestCountry=@guestcountry;

应该：

strCommandText +=" WHERE Guest.RoomID=Room.RoomID AND Guest.GuestPassportNo=@guestpno AND (HotelPackages.PackageID=Guest.HotelPackageID OR GUEST.HotelPackageID IS NULL) AND Guest.GuestCountry=@guestcountry;