If Username Exists, Return Try Again Otherwise Write To Db

I am a SQL newbie trying to create a registration page. The registration data is stored in "tblUsers," and the username is stored in the field named "UID" -- which is the primary key. If the user enters a username that already exists in the database, I want to return the "That username already exists" message and keep them on the registration page. If the username doesn't exist, I want to write all their registration info to the database.

I tried the code below using both an existing username and then using a username that didn't exist. Both times I got the "that username already exists" error message on the top of the confirmation page, and the record was not written to the database.

I also tried changing the "If objRec.RecordCount <> 0" to "If objRec.RecordCount >0" I got it to write to the database if the username didn't exist, but if the username did exist, I got the microsoft error '80040e14' (The changes you requested to the table were not successful because they would create duplicate values...)

Thanks for any help.


<%
SQLCmd = "SELECT UID From tblUsers WHERE UID = " & "'" & UID & "'"
Set objRec = objConn.Execute(SQLCmd)
If objRec.RecordCount <> 0 Then
Response.write "That username already exists"
Else
SQLCmd = "INSERT INTO tblUsers"
SQLCmd = SQLCmd & " (FIRSTNAME, LASTNAME, UID, PWD, STREET, CITY, STATE, ZIP, EMAIL, PHONE, FAX, PHYSICIAN, SPECIALTY, OTHER) "
SQLCmd = SQLCmd & " VALUES ('" & firstname & "',"
SQLCmd = SQLCmd & "'" & lastname & "',"
SQLCmd = SQLCmd & "'" & UID & "',"
SQLCmd = SQLCmd & "'" & PWD & "',"
SQLCmd = SQLCmd & "'" & street & "',"
SQLCmd = SQLCmd & "'" & city & "',"
SQLCmd = SQLCmd & "'" & state & "',"
SQLCmd = SQLCmd & "'" & zip & "',"
SQLCmd = SQLCmd & "'" & email & "',"
SQLCmd = SQLCmd & "'" & phone & "',"
SQLCmd = SQLCmd & "'" & fax & "',"
SQLCmd = SQLCmd & "'" & physician & "',"
SQLCmd = SQLCmd & "'" & specialty & "',"
SQLCmd = SQLCmd & "'" & other & "')"
Set objRec = objConn.Execute(SQLCmd)
End If
%>