C#判断select语句返回是否为空问题

首先我想到是根据SqlCommand.ExecuteNonQuery 方法返回的int值进行判断，但是在MSDN上说ExecuteNonQuery返回受影响的行数，需要注意的是受影响的行数是执行
UPDATE、INSERT 或 DELETE 语句的结果，对于select语句它总是返回-1。。。

所以最好用SqlDataReader来实现，代码如下(注意一定要关闭SqlDataReader)：

SqlCommand myCommand = cn_sql.CreateCommand();
myCommand.CommandText = "Select Teacher from Login where Teacher = '" + user + "'";
SqlDataReader myReader = myCommand.ExecuteReader();
if(myReader.Read())
{
myReader.Close();
myCommand.CommandText = "update Login set Lesson_ = '" + lessonid + "' where Teacher = '" + user + "'";
myCommand.ExecuteNonQuery();
}
else
{
myReader.Close();
myCommand.CommandText = "insert into Login VALUES ('" + user + "','" + lessonid + "')";
myCommand.ExecuteNonQuery();
}