Max的Linq分组不起作用

Question

I have a table with products

Id  |  Name      |  Order
1   | product 1  |    5
1   | product 1  |    9
1   | product 1  |    2 
2   | product 2  |    0
3   | product 3  |    1

I need to return just the product with the max order number:

Id  |  Name      |  Order

1   | product 1  |    9
2   | product 2  |    0 
3   | product 3  |    1

tried this but it's not working:

var items = (from i in db.products
                 group i by new
                 {
                     i.Id,
                     i.Name,
                     i.Order
                 } into g

                 select new
                 {
                     g.Key.Id,
                     g.Key.Name,
                     Order = g.Where(d => d.Order == g.Max(xx => xx.Order )).First().Order     })

but it stills return all the 5 results. thank you

Answer 1

Change

             group i by new
             {
                 i.Id,
                 i.Name,
                 i.Order
             } into g

into

             group i by new
             {
                 i.Id,
                 i.Name,
             } into g

I think I would also rewrite the the last expression to

Order = g.OrderByDescending(xx => xx.Order).First().Order

Mostly because I think it is easier to read, but the generated SQL might be more efficient too. But this is just a matter of taste and any performance difference can be verified with SQL Profiler.

Answer 2

The reason you still have 5 rows is because Order is different for your group by combination. This would be the query if you wanna use the original one, or use the order by to get the max Order.

var items = (from i in db.products
                 group i by new
                 {
                     i.Id,
                     i.Name                     
                 } into g

                 select new
                 {
                     g.Key.Id,
                     g.Key.Name,
                     Order = g.Where(d => d.Order == g.Max(xx => xx.Order )).First().Order     })