MySQL在一个查询中计算多列

Question

I've been using MySQL for just a day to handle my login system and i've come across a problem when creating a new user. I've got 3 fields i need to worry about, Display name, Username and Email . Now i need to check if any of the fields already have an entry in the database. So if the display name that has been given is already in the database then it should show a message " This display name is already taken" and the same for the other 2.

I've already read countless posts where people use SELECT COUNT and tons more but i still haven't been able to fit it in my own project.

How would i go about doing this?

Answer 1

I was trying for way too long to find myself a solution for this, after a lot of trail and error i've made myself this bit of code. It might not be perfect but it works just fine:

string sql = "SELECT Nickname, Username, Email " 
        + " FROM UserData WHERE (Nickname=?Nickname OR Username=?Username OR  Email=?Email)";

MySqlCommand check = new MySqlCommand(sql, conn);
conn.Open();
check.Parameters.AddWithValue("?Nickname", txtCreateName.Text);
check.Parameters.AddWithValue("?Username", txtCreateUserName.Text);
check.Parameters.AddWithValue("?Email", txtCreateEmail.Text);
MySqlDataReader Reader;
Reader = check.ExecuteReader();

List<string> Nicknames = new List<string>();
List<string> Usernames = new List<string>();
List<string> Email = new List<string>();

while (Reader.Read())
{
    Nicknames.Add(Reader.GetString(0));
    Usernames.Add(Reader.GetString(1));
    Email.Add(Reader.GetString(2));
}
conn.Close();
Reader.Close();
Boolean alreadyexists = false;


if (Nicknames.Contains(txtCreateName.Text))
{
    alreadyexists = true;
    lblErrorDisplay.Visible = true;
}
if (Usernames.Contains(txtCreateUserName.Text))
{
    alreadyexists = true;
    lblErrorUsername.Visible = true;

}
if (Email.Contains(txtCreateEmail.Text))
{
    alreadyexists = true;
    lblErrorEmail.Visible = true;
}
if (alreadyexists)
{
    return;
}

If anyone has some improvements on this code it'll be much appreciated but this'll work for now.

Answer 2

Just create UNIQUE INDEX on each of those fields. That way you can be sure there wouldn't be any duplicates, because DB handle that internally. And also no race condition.

To handle duplicate insert, just check for specific error code.