c# mysql sum query to label

问题

I'm trying to do something I feel is quite simple in the grand scheme of things, however I'm clearly missing something. What I have is a simple database named 'localcollection'. What I would like to do is sum up the dollar amount of a column named 'purprice', and setting it as the text of a label (label4). I've been finding variants of code throughout the last couple days that suggest different ways of achieving this. The majority of my digging suggest that using ExecuteScalar is what I want to do. The code that I've been fumbling with follows.

SqlCeConnection myconn = new SqlCeConnection(Properties.Settings.Default.localbotdbConnectionString);
myconn.Open();

{
    string result = "select sum(purprice) from localcollection";
    SqlCeCommand showresult = new SqlCeCommand(result, myconn);

    label4.Text = showresult.ExecuteScalar().ToString();
    myconn.Close();
}

Others suggest using the SqlCeReader. I'm impartial to either of them, as long as one of them works, and I am clearly missing something (fault of my own). The reader rendition that I was trying to make work is:

SqlCeCommand cmd = new SqlCeCommand("select sum(purprice) from localcollection");
SqlCeDataReader reader = cmd.ExecuteReader();
while(reader.Read())
{
    label4.Text = reader.GetString(0);
}
myconn.Close();

Constructive advice appreciated. Thank you

回答1:

I would do some modifications to your code because for one thing your are not properly disposing of your objects, also you stated that you have it in the button click method which I would get that out of there and make this its own function.

private string performSQL() 
{
   string result = "select sum(purprice) from localcollection";
   using (SqlCeConnection myconn = new SqlCeConnection("ConnectionString"))
   using (SqlCeCommand showresult = new SqlCeCommand(result, myconn))
   {
      try
      {
          myconn.Open();
          return showresult.ExecuteScalar().ToString();

      }catch(System.Exception ex)
      {
          MessageBox.Show(ex.ToString());
          // or log exception how ever you prefer
      }finally
      {
          //the finally ensures your connection gets closed
          myconn.Close();
      }
   }
   return "";
}

回答2:

If you are only looking to return one value from a query, then ExecuteScalar is what you should be using, . The ExecuteReader is better for forward-only reads of multiple records, so it's overkill for your situation

Take a look here for a comparisson What is the difference between ExecuteScalar, ExecuteReader and ExecuteNonQuery?

回答3:

Best practice, use ExecuteScalar when you are returning 1 row and 1 column of data (which your query does.) As a result, go with ExecuteScalar.

回答4:

Make sure the name of the column you are trying to add is purprice, and that it is a numeric type. Also make sure it doesn't contain NULL values.

回答5:

I think your code is ok, but you missed :

  SqlCeCommand cmd = new SqlCeCommand("select sum(purprice) from localcollection",myconn);

that's all, hope it works

回答6:

Just add AS after SUM() like the row below:

select sum(purprice) AS purprice from localcollection

And you are good to go.

来源：https://stackoverflow.com/questions/17834927/c-sharp-mysql-sum-query-to-label

标签

mysql

sum

label