我在进行计算时无法保留该值 [英] Cannot keep the value when I do the calculation

问题描述

我的ReservationID就像这个R00001但是当自动生成ReservationID时，值0不能保留在变量中。如何解决？

My ReservationID like this R00001 but when auto generate the ReservationID, the value 0 cannot keep in the variable. How to solve it ?

public void GenerateReservationID()
  {
      string sql = "SELECT * FROM Reservation";
      string cs = @"Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\Database.mdf;Integrated Security=True;User Instance=True";
      string s = "";

      SqlConnection con = new SqlConnection(cs);
      SqlCommand cmd = new SqlCommand(sql, con);

      con.Open();
      SqlDataReader dr = cmd.ExecuteReader();
      while (dr.Read())
      {
          s = Convert.ToString(dr["ReservationID"]);
      }
      string sub1 = s.Substring(0,1);
      string sub2 = Convert.ToString(Convert.ToInt16(s.Substring(1)) + 1);
      dr.Close();
      con.Close();

      lblRservationID.Text = sub1 + sub2;
  }

[edit]已添加代码块 - OriginalGriff [/ edit]

[edit]Code block added - OriginalGriff[/edit]

推荐答案

好的，这里有几个问题：

1）你读取所有行，并且只使用返回的最后一个值（因为循环丢弃了所有其他值） - 但是你没有指定一个订单，所以SQL可以按照它认为合适的任何顺序 高兴地返回行 。它们不一定按照输入的顺序返回。

另外，返回你不需要的数据是一个坏主意 - 它会大大减慢速度，并浪费大量的内存/ SQL服务器时间/带宽。将SELECT仅与您实际需要的数据一起使用。

考虑仅返回您感兴趣的行：

OK, there are a couple of problems here:
1) You read all the rows, and only use the last value returned (because the loop throws away all the other values) - but you don't specify an order, so SQL can happily return the rows in any order it sees fit. They will not necessarily be returned in the order they were entered.
In addition, it is a bad idea to return data you don't need - it can slow things down considerably, and waste a lot of memory / SQL server time / bandwidth. Use SELECT only with the data you actually want.
Consider returning only the row you are interested in:

SELECT MAX(ReservationID) FROM Reservation 

将仅返回当前最高的数字，这是我怀疑你的感兴趣。



2）当你将一个字符串转换为一个整数时，你不会丢失零 - 但是将一个整数转换回来的常规方法字符串不会插入前导零 - 这就是你得到的效果。对于一个整数，00000001与1相同，因为前导零完全不相关。

尝试使用ToString格式来指定前导零，或者String.Format：

Will return only the current highest number, which is what I suspect you are interested in.

2) When you convert a string to an integer, you don't "lose" zeros - but the normal method to convert an integer back to a string does not insert leading zeros - so that is the effect you get. To an integer, 00000001 is the same as 1 because leading zeros are completely irrelevant.
Try using ToString with a format to specify the leading zeros instead, or String.Format:

string sub1 = s.Substring(0,1);
int reserveNp = Convert.ToInt32(s.Substring(1)) + 1;
lblReservationID.Text = string.Format("{0}{1:00000}", sub1, reserveNo);

3）虽然还有另一个问题：由于你使用SQL，很有可能在多用户环境中使用它，你在这里使用的想法是行不通的：如果两个用户同时去预订号码，那就有一个，很有可能他们都会得到相同的价值。

你在哪里创建这个预订ID，为什么不从那里保留价值？

3) There is another problem though: Since you are using SQL there is a good chance that this will be used in a multi user environment, where the idea you are using here will not work: if two users go for a reservation number at the same time, there is a very, very good chance that they will both get the same value.
Where are you creating this reservation ID, and why aren't you keeping the value from there?

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