How to select columns after group by in entitiy frame - sql

Question

I am trying to select multiple columns after groupby operation in EF.
Here is what I want to do:
this._context.dbtablename
.Select(u => new { u.name, u.age, u.grade })
.GroupBy(x => x.grade)
.Select(g => new { name = g.?, age = g.??, grade = u.key, Count = g.Count() })
Example:
name age grade
A 12 8
B 12 9
C 11 10
D 11 9
I want to have:
name age grade
A 12 8
B 12 9 ----> "name" is B or D, "age" is "12" or "11", does not matter.
C 11 10
Can I achieve it?

Answer 1

each grouped item has a key and collection so you can use first() or firstordefault() to get first item in each group
this._context.dbtablename
.Select(u => new { u.name, u.age, u.grade })
.GroupBy(a => a.grade)
.Select(a => new { name = a.First().name, age = a.First().age, grade = a.Key, count = a.Count() });
or min() or max()
.Select(a => new { name = a.Min(b => b.name), age = a.Min(b => b.age), grade = a.Key, count = a.Count() });

Answer 2

Following returns number of valid orders (where clause) in periods (YearMonth YYYYMM):
GM_ORDER.AsQueryable()
.Where(s => s.STATUS > -1)
.Select(s => new { DateOfOrder = s.CREATEDON.Date, s.ORDERID })
.GroupBy(g => g.DateOfOrder.Year*100 + g.DateOfOrder.Month )
.OrderByDescending(o=>o.Key)
.Select(g => new { Period = g.Key, NrOfOrders = g.Count()} )
Output is:
Period NrOfOrders
201705 13
201704 34
201703 42
201702 17
201701 17
201612 25
201611 34
201610 54
201609 135
201608 163
201607 110
201606 169
201605 100
201604 56
201603 19
201602 8
201601 6
Hope this helps.
Edited:
If you like to see more columns from the grouping following will help. To sort, you need to take it into a list variable and then sort the list variable in itself.
GM_ORDER.AsQueryable()
.Where(s => s.STATUS > -1)
.Select(s => new { DateOfOrder = s.CREATEDON.Date, s.ORDERID, s.STATUS })
.GroupBy(g => new { YYYYMM = g.DateOfOrder.Year * 100 + g.DateOfOrder.Month, g.STATUS })
.Select(g => new { Period = g.Key.YYYYMM, Status = g.Key.STATUS, NrOfOrders = g.Count() })
.ToList()