从MS Access获取记录

Question

运行我的代码后，它会显示以下消息：

  

此子查询最多可以返回一条记录。

那么，我怎样才能得到所有的鱼尾？

这是我的代码：

  connection.Open();
            OleDbCommand command1 = new OleDbCommand();
            command1.Connection = connection;
            string cq = "Select Email From student where S_ID=(select S_ID FROM studentbook where DateDiff('d',[Issue_Date], NOW())=31) ";
             command1.CommandText = cq;
             OleDbDataAdapter da1 = new OleDbDataAdapter(command1);
             DataTable dt1 = new DataTable();
             da1.Fill(dt1);
            foreach (DataRow row in dt1.Rows)
            {
                string email = row["Email"].ToString();
                MessageBox.Show("Trying to send email ");

                using (MailMessage mm = new MailMessage("sujitcsecuet@gmail.com", email))
                {
                    mm.Subject = "Attention please,renew your book";
                    mm.Body = string.Format("1 month over,you should renew or return the book");

                    mm.IsBodyHtml = true;
                    SmtpClient smtp = new SmtpClient();
                    smtp.Host = "smtp.gmail.com";
                    smtp.EnableSsl = true;
                    System.Net.NetworkCredential credentials = new System.Net.NetworkCredential();
                    credentials.UserName = "abc@gmail.com";
                    credentials.Password = "password";
                    smtp.UseDefaultCredentials = true;
                    smtp.Credentials = credentials;
                    smtp.Port = 587;
                    smtp.Send(mm);
                    MessageBox.Show("Email sent successfully");
                }
            }