Question

I am writing a program in C#.NET
I would like to gather the total frequency of a class (let imagine every class has many words and each word has its own frequency in the corresponding class)

So i used the sum() function in mysql. But there is an error saying that my cast is wrong.

 public void average_each_type()
        {
            MySqlDataReader result;
            int total_freq = 0;
            string type = "";

            command.CommandText = "select class_name ,SUM(frequency) as sum_of_freq from training_set group by class_name ";


                result = command.ExecuteReader();
                while (result.Read())
                {

                    total_freq = (int)result["sum_of_freq"]; //error happened here
                    type = result["class_name"].ToString();
                    //.....then so on...//

Answer 1

SUM in MySql will return a decimal or double value, depending on the type within “frequency”. From the documentation for SUM:

The SUM() and AVG() functions return a DECIMAL value for exact-value arguments (integer or DECIMAL), and a DOUBLE value for approximate-value arguments (FLOAT or DOUBLE). (Before MySQL 5.0.3, SUM() and AVG() return DOUBLE for all numeric arguments.)

If you want an integer, you can use Convert to get one no matter what the source type happens to be:

total_freq = Convert.ToInt32(result["sum_of_freq"]);

The advantage here is the Convert.ToInt32 will work no matter what type of value is returned from the database, provided it is a numeric type.