尝试一下
检查用户可用性

Try this
Check user availability

'START .
        Dim conn = New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;" & _
    "Data Source=" & Application.StartupPath & "\DBase.accdb;")
        conn.Open()
        Dim Str As String
        Str = "SELECT * FROM DB WHERE [Username] = '" & UsernameTextBox.Text & "'"
        Dim cmmd = New OleDbCommand(Str, conn)
        Dim dr As OleDbDataReader
        dr = cmmd.ExecuteReader()

        If Not dr.HasRows Then
            MsgBox("Username available.")
            'REST OF THE PROGRAM

        Else
            MsgBox("Username already taken. Try another.")
        End If

        'END