如何检查值是否已经存在

Question

I am trying to find a lot of time how to check if value exists and I can not find it, I need to check it twice:

1. in the sign up
2. in the log in

Here is my code block:

SqlConnection c = new SqlConnection(str);
        SqlCommand cmdUsername = new SqlCommand("SELECT 1 FROM Users WHERE UserName = @userName;", c);
        cmdUsername.Parameters.AddWithValue("userName", userName);
        cmdUsername.CommandType = System.Data.CommandType.Text;
        SqlCommand cmdEmail = new SqlCommand("SELECT 1 FROM Users WHERE Email = @email;", c);
        cmdUsername.Parameters.AddWithValue("email", email);

        c.Open();
        nameExists = (int)cmdUsername.ExecuteScalar();
        emailExists = (int)cmdEmail.ExecuteScalar();
        c.Close();

When I am entering an email it marks the line

emailExists = (int)cmdEmail.ExecuteScalar();

And in the log in all is ok.

Please help me! Thank you all.

Answer 1

Assuming you want to prevent duplicate User Name or Email you have to do the following. 1. Set the ID column as INT and set it to Identity column from column properties. 2. Set your Email or User Name as primary key to prevent either Email or User Name from duplication, The best practice is to make the Email column as primary key where there are a lot of cases that the users have same name but with unique emails.

Hope that helps!

And to check whether username or email exist already here how you do it!

SqlCommand check_User_Name = new SqlCommand("SELECT COUNT(*) FROM [Table] WHERE ([user] = @user || [email] = @email)" , c);
check_User_Name.Parameters.AddWithValue("@user", txtBox_UserName.Text);
check_User_Name.Parameters.AddWithValue("@email", txtBox_Email.Text);
int UserExist = (int)check_User_Name.ExecuteScalar();

if(UserExist > 0)
{
   //Username exist
}
else
{
   //Username doesn't exist.
}

Answer 2

Using your code, just optimized it a bit.

• Just one call to database
• Always close the connection even if a exception occurs
• Returns false when not found or na exception occurs, true otherwis

You can also add a check to the password if this is a login check.

public bool ValidData(string username, string email, string connectionString)
{
    var c = new SqlConnection(connectionString);
    var cmdUsername = new SqlCommand("SELECT COUNT(*) FROM Users WHERE UserName = @userName AND email = @email;", c);
    cmdUsername.Parameters.AddWithValue("userName", username);
    cmdUsername.CommandType = System.Data.CommandType.Text;
    cmdUsername.Parameters.AddWithValue("email", email);

    c.Open();
    try
    {
        return (int) cmdUsername.ExecuteScalar() > 0;
    }
    catch (Exception ex)
    {
        //log exception

        return false;
    }
    finally
    {
        c.Close();
    }
}

If you just need the username or the email change:

    var cmdUsername = new SqlCommand("SELECT COUNT(*) FROM Users WHERE UserName = @userName AND email = @email;", c);

to:

    var cmdUsername = new SqlCommand("SELECT COUNT(*) FROM Users WHERE UserName = @userName OR email = @email;", c);

Answer 3

Try this -

string myConnection=str;//this is your connection string
string userName="";
string email="";
string parameteruserName = "";//your parameter should goes here
string parameterEmail = "";//your parameter should goes here
try
{
    SqlDataReader myReader = null;
    SqlCommand    myCommand = new SqlCommand(SELECT COUNT(*) as count FROM  Users WHERE UserName =" +parameteruserName+" or  Email ="+parameterEmail 
   )  "+  ";", 
                                             myConnection);
    myReader = myCommand.ExecuteReader();
    while(myReader.Read())
    {
      userName=  myReader["count"].ToString();

    }
myReader.close();
myConnection.Close();
}
catch (Exception e)
{
    Console.WriteLine(e.ToString());
}


if(Convert.ToInt32(userName)>0){
//your user name is exists
}