使用C＃检查用户是否已存在于MySql数据库中

Question

I have the following code and I would like the code to query the database:

 MySqlConnection connection = new MySqlConnection(Connection);
 MySqlCommand cmd;
 connection.Open();
  try
  {
    cmd = connection.CreateCommand();
    cmd.CommandText = ("INSERT INTO edata (username) VALUES ('" + this.NewUserName.Text +"') ;");
    MySqlDataReader myReader;
    myReader = cmd.ExecuteReader();
    loading.Show();
    MessageBox.Show("Your Account Has Been Created!!");
    connection.Close();
  }
  catch (Exception ex)
  {
    MessageBox.Show(ex.Message);
  }

If the clients input (NewUserName.Text) is the same as the username in the database there should be an error message, otherwise there will be duplicate usernames. How can I do this?

I have already got the connection as a string.

Thank you in advance!!!

PS Im a student learning C# and MySql, I am grateful for any help and advice!

Answer 1

You can create a Unique Constraint on the username field in the database, this will throw an exception if the user already exists.

Alternatively, you do a manual select from the table and see if there is a user with the same name before insertion.

Answer 2

您可以在适当的列（ doc ）上创建唯一索引。

CREATE UNIQUE INDEX uq_username ON edata (username);

Answer 3

are you using the membership class?

      MembershipCreateStatus createStatus;
        MembershipUser newUser = Membership.CreateUser(txtEmail.Text, "password", txtEmail.Text,"What was your first school?","idk", true, out createStatus);


            switch (createStatus)
        {
            case MembershipCreateStatus.Success:
           //do your actions here
            case MembershipCreateStatus.DuplicateUserName:
                CreateAccountResults.Text = "There already exists a user with this username.";
                break;

            case MembershipCreateStatus.DuplicateEmail:
                CreateAccountResults.Text = "There already exists a user with this email address.";
                break;
            case MembershipCreateStatus.InvalidEmail:
                CreateAccountResults.Text = "There email address you provided in invalid.";
                break;
            case MembershipCreateStatus.InvalidAnswer:
                CreateAccountResults.Text = "There security answer was invalid.";
                break;
            case MembershipCreateStatus.InvalidPassword:
                CreateAccountResults.Text = "The password you provided is invalid. It must be seven characters long and have at least one non-alphanumeric character.";

                break;
            default:
                CreateAccountResults.Text = "There was an unknown error; the user account was NOT created.";
                break;
        }

If you're not using that, then simply create a method that reads the database and call it before the code you have above:

string doesuserexist = ReadDatabase("Select Email from Patients where Email=@emailaddress").TrimEnd();
            if (checkemail != "")
            {
                CreateAccountResults.Text = "User Email Already Exists";
                return;
            }

You have to write your own ReadDatabase method, but from the code posted above, you obviously know how, just replace the insert sql statement with select.