string s = "server=.;database=SampleDb;Integrated Security=True";
            SqlConnection c = new SqlConnection(s);
            c.Open();

            SqlCommand command = new SqlCommand();
            string sql="select count(*) from student"
            command.Connection = c;
            command.CommandText = sql;

           int n = command.ExecuteScalar();
            c.Close();
           Messagebox.show("共有"+n+"个学生");