SQL to LINQ C# with join max and group

Question

How can I write the following query in LINQ.

select 
    a.IntId, Max(b.CreateDate), Max(a.TwoID) 
from 
    TableA a 
join 
    TableB b on a.IntId = b.IntId 
group by 
    IntId

I've got this currently but it doesn't work:

var res = DbContext.TableA
                   .Include(c => c.TableB)
                   .Select(p => new { CreateDate = p.TableA.CreateDate, IntId = p.IntId, TwoID = p.TwoID })
                   .GroupBy(x => x.IntId)
                   .Select(grp => grp.OrderByDescending(x => x.CreateDate)
                   .First()).ToList();

Answer 1

Your identifiers are not very meaningful. You did this probably for the question, but it makes it fairly difficult to grasp the relation between TableA and TableB .

It seems to me that there is a one-to-many relation between TableA and TableB: every row from TableB has zero or more rows from TableA; every row from TableA "belongs" to exactly one row from TableB, namely the row that foreign key IntId refers to.

From every TableB row with its TableA items , you want to fetch TableB.IntId , TableB.CreateDate and the largest TableA.TwoId of all TableA rows that belong to this TableB.

Whenever you need to fetch "items with their sub-items" from a one-to-many relation, like Schools with their Students, Customer with their Orders, or "TableB rows with their TableA rows", consider to use one of the overloads of Queryable.GroupJoin .

Because I don't just want some special results, not just "TableB rows with their TableA rows", I use the overload that has a parameter resultSelector:

IQueryable<TableARow> tableA = ...
IQueryable<TableBRow> tableB = ...

var result = tableB.GroupJoin(tableA,   // GroupJoin TableB with TableA

    tableBRow => tableBRow.IntId,       // from every row in tableB take the IntId
    tableARow => tableARow.IntId,       // from every row in tableA take the foreign key

    // parameter resultSelector: from every row in tableB, with its zero or more 
    // matching rows from tableA, make one new object:
    (tableBRow, matchingTableARows) => new
    {
        IntId = tableBRow.IntId,
        CreateDate = tableBRow.CreateDate,

        LargestTwoId = matchingTableARows.Select(tableARow => tableARow.TwoId).Max(),
    })

In words: from every row from tableB, take the value of IntId. Then find all rows from tableA that have a matching IntId value. From every tableB row, with all its zero or more tableA rows, create one new object with the following three properties:

• IntId is the IntId of the tableBRow (which, by the ways equals the IntId of all matching tableARows)
• CreateDate is the CreateDate of the tableBRow
• LargestTwoId is calculated as follows:

from every tableARow that has a value for IntId that matches the tableBRow, take only the value of property TwoId. From all these TwoId values take the largest one.

I chose to use Max instead of OrderByDescending , because it is a waste of processing power to Order all elements, if you will be using only the first one.

There is a small flaw with this: Max only works if there is at least one element. If you think there are some tableBRows that have no matching tableARow, then this won't work.

LargestTwoId = matchingTableARows.Select(tableARow => tableARow.TwoId)
    .OrderByDescending(twoId => twoId)
    .FirstOrDefault(),

or:

LargestTwoId = matchingTableARows.Any() ?
   matchingTableARows.Select(tableARow => tableARow.TwoId).Max() :
   null,