LINQ - Summing numbers stored as string

问题

I have the following data:

PK    OrderNumber      USERDEFFIELD
1     0001             10
2     0001             25
3     0002             20
4     0002             22
5     0002             NULL
6     0003             ABC123

The UserDefField column is of VARCHAR type in the database. Using LINQ, how can I get the SUM(UserDefField) per order? NULL and non-numeric values for UserDefField are to be considered as zero. The result I'm trying to get:

OrderNumber  TotalQty
0001         35
0002         42
0003          0

If UserDefField is strictly nullable numeric field I know I would do this inside a foreach loop:

 TotalQtyForThisOrder = orders.Sum(w => w.UserDefField ?? 0 );

But for a string field, what should be done? Thank you very much for the help.

回答1:

TotalQtyForThisOrder = orders.Sum( w = > {
            int result;
            Int32.TryParse(w.UserDefField, out result);
            return result;
});

回答2:

Fundamentally, I'd say your schema is off: if you're treating a textual value as a number if it happens to be parsable that way, you're really fighting against sensible design.

I don't know how you'd be able to write a LINQ query to make that sum occur in the database. You'd probably want to write some custom conversion function, or perhaps a custom view which did the "try to parse it and return 0 otherwise" operation.

You can do it easily enough within LINQ to Objects:

x = orders.Select(y => y.UserDefField) // To just bring down the right values
          .AsEnumerable()
          .Sum(text => {
                   int result;
                   // We don't care about the return value... and
                   // it even handles null input. (In any error condition,
                   // "result" will be 0 afterwards.)
                   int.TryParse(text, out result);
                   return result;
               });

... but I'd really recommend that you revisit your schema if you possibly can.

回答3:

I would use the Aggregate function like so:

var result = orders.Aggregate(new Dictionary<OrderIdType, int>(), (accumulator, item) => {
    int quantity;
    if (!int.TryParse(item.USERDEFFIELD, out quantity))
        quantity = 0;
    if (!accumulator.ContainsKey(item.OrderId))
       accumulator[item.OrderId] = quantity;
    else
       accumulator[item.OrderId] += quantity;
    return accumulator;
});

来源：https://stackoverflow.com/questions/8917303/linq-summing-numbers-stored-as-string

标签

linq

entity-framework-4

linq-to-entities