Identify firstname of employee when RFID card is swiped and insert into database

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SQL

I have been trying to resolve this for quite some time now but not achieved yet. My problem is i am trying to insert values of my fields(firstname,rfid,sdt(datetime)) into datatab and display a report of that data.

now the scenario is rfid card can be swiped a number of times throughout the day by the employee i am storing that data into my admin table and i have successfully accomplished displaying and storing rfid and sdt fields into the table but firstname must be fetched from table Admin in which unique tag of rfid at the time of registration of employee is assigned to them.

I have tried joins,multiple queries into one but nothing worked for me so far. query to store those two values is :

What I have tried:

SqlConnection con = new SqlConnection(str);
con.Open();
SqlCommand cmd = new SqlCommand("Insert into Datatab(firstname) Select Distinct firstname from Admin", con);
cmd.Parameters.AddWithValue("@firstname", txtname.Text);
//       cmd.Parameters.AddWithValue("@rfid", txtname.Text);
cmd.ExecuteNonQuery();
con.Close();

I want the name of the specific employee who swipes his/her card for n no of times with rfid of that employee to be saved into the database.