SqlCeConnection sqlCE = new SqlCeConnection(@"Data Source=C:DBAvtoBazar.sdf;");

SqlCeConnection.Open();

string sqlQueryString = "Select NameAvto From Avto"; //для конкретного значения добавь Where
SqlCeCommand comand = new SqlCeCommand(sqlQueryString, sqlCE);
SqlCeDataReader dataReader = comand.executeReader();

List<string> avtoNames = new List<string>();
while(dataReader.Read()){
  avtoNames.add(dataReader.getString(0));
}
dataReader.close();

using (SqlConnection conn = new SqlConnection(@"Data Source=YOURIP;Initial Catalog=YourCatalog; Integrated Security=no; User id = Name; Password = Pass;"))
            using (SqlCommand command = new SqlCommand("SELECT ID FROM Us where Username='Q'"))
            {

                command.Connection = conn;
                conn.Open();
                SqlDataReader reader = command.ExecuteReader();
                if (reader.Read())
                {
                    Int32 ret = reader.GetInt32(0);
                    Console.WriteLine(ret.ToString());
                }
            }