how to match the data from database without post back like compare validater ?

See more:

VB

ASP.NET

i want to match the username from database ?
without post back like 'odesk.com'

i want to use this like compare validater
any one can help me please ?

Protected Sub btnSignup_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btnSignup.Click
       con.Open()

       com.Connection = con
       com.CommandText = "SELECT userName FROM signUp WHERE userName =@userName"
       com.Parameters.Add(New SqlClient.SqlParameter("@userName", txtUsername.Text))
       Dim str As String = com.ExecuteScalar
       If str = "" Then

           lblusernametaken.Visible = False
       Else
           lblusernametaken.Visible = True
           Exit Sub
       End If
End sub 

i want to remove this button click event
plz help me


thank you