确定用户登录名是否已经存在于数据库中？

Question

I am building a web application in asp.net using C#. I have the Form where the user should register and then can login. I am having a problem in making the web app know that the name which the user picks is either "already exists" or not. If it already exists it should not insert the same name and display a message saying "user name already exists". I have tried the SqlDataReader but no luck.

protected void Register_Button_Click(object sender, EventArgs e)
    {
        SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["BJ_Player_String"].ToString());
        SqlCommand cmd = new SqlCommand();
        SqlCommand cmd2 = new SqlCommand();
        SqlDataReader data_reader;
        String name = TextBox2.Text;
        String date = TextBox3.Text;


        try
        {
            conn.Open();
            cmd = new SqlCommand("Insert into BJ_Player (Player_Name, D_O_B) Values (@Player_name, @D_O_B)", conn);
            cmd = new SqlCommand("Select Player_Name from BJ_Player WHERE Player_Name = @Player_name", conn);
            cmd.Parameters.Add("@Player_name", SqlDbType.NVarChar).Value = name;
            cmd.Parameters.Add("@D_O_B", SqlDbType.Date).Value = date;
            cmd.Connection = conn;
            data_reader = cmd.ExecuteReader();
            cmd.ExecuteNonQuery();

            if (data_reader.HasRows)
            {
                lblPlayerNameExists.Visible = true;
            }
            else
            {
                // do nothing
            }    
        }

Answer 1

Make Player_Name unique in database then it will give you exception when you try to insert. You have to use unique constraint. You have to give command type also and check you assigned both queries to same cmd object

Answer 2

in your code you're inserting data in your DB and then you are examining that the name is the same or not. first you should search the name in your DB and then if there isn't any record with that name ,you should add your record.

Answer 3

If you would like to write a model function to do that then

1. Leave it for ajax check which is pretty similar to the second method
2. Issue "Select username from DB-table" to retrieve usernames then check the username input against them before displaying a view to report a problem if any or showing a message to tell the user that "this name is valid", for example.

Answer 4

I usually do it in one of two ways:

1. Create stored procedure that will check for name uniqueness and insert new record if everything is ok. It should return status as numeric code that you will check.
2. Check for name uniqueness before saving it using as a part of validation process.

Answer 5

Using the merge statement may help with this. Merge performs insert, update, or delete operations on a target table based on the results of a join with a source table.

Basically it inserts when needed, and updates when needed. Often times referred to as an upsert. but it gets the job done.

Here is a link to a site explaining how to use merge. Looks like a good article. http://www.kodyaz.com/articles/sql-server-2008-t-sql-merge-statement-example.aspx