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How to shorten the LINQ query to calculate the sum for parent table from child tables

Currently I have this LINQ query which calculate the totalcount of a parent table (CafeTables) from child tables (CafeTableDetails). These code works. But somehow, I believe these code can the shorten.
var selectedTable = db.CafeTables.Where(c => c.TableNo.Equals(userName)).SingleOrDefault();
var selectedTableDetailsRaw = db.CafeTableDetails.
Where(cd => cd.CafeTableId == selectedTable.Id);
selectedTable.TotalOrders = selectedTableDetailsRaw.Count();

I think you can try to use linq JOIN and GroupBy to make it.
var result = db.CafeTables.Where(c => c.TableNo == userName)
.Join(db.CafeTableDetails.GroupBy(x=>x.CafeTableId)
.Select(g => new { CafeTableId = g.Key, cnt = g.Count() }),
st => st.Id,
cd => cd.CafeTableId,
(st,cd) => new
{
st.Id,
cd.cnt
//..... your expect property
});

Transform SQL Query to LINQ

I would like if someone helps me to convert this SQL Query to LINQ syntax.
SELECT i.Id, i.Condomino as Condomino, i.Interno as Interno,
p.NomePiano as NomePiano, s.Nome as NomeCondominio,
m.millesimi_fabbisogno_acs, m.millesimi_fabbisogno_riscaldamento
FROM Interni i
INNER JOIN Piani p ON i.IdPiano = p.Id
INNER JOIN Stabili s ON i.IdStabile = s.Id
LEFT JOIN MillesimiTabellaC m ON i.Id = m.idInterno
WHERE s.IdCondominio = {0}
I tried using something like this, but is not working..
return _Db.Interni.Include("Piani").Where(x => x.Piani.IdCondominio == iidcond).ToList();

I made it on-the-spot (so it's not tested), but perhaps it's enough to give you the idea. I'm also assuming that your DB model has foreign keys set up.
var result = _db.Interni
.Where(i => i.Stabili.IdCondominio = [value])
.Select(i => new
{
i.Id,
Condomino = i.Condomino,
Interno = i.Interno,
NomePiano = i.Piani.NomePiano,
NomeCondominio = i.Stabili.Nome,
i.MillesimiTabellaC.millesimi_fabbisogno_acs,
i.MillesimiTabellaC.millesimi_fabbisogno_riscaldamento
})
.ToList();
update
In case you don't have a foreign key between Interni and MillesimiTabellaC, try this:
var result = _db.Interni
.Include(i => i.Piani)
.Include(i => i.Stabili)
.Where(i => i.Stabili.IdCondominio = [value])
.Select(i => new
{
Interni = i,
MillesimiTabellaC = _db.MillesimiTabellaC.Where(m => i.Id = m.idInterno)
})
.Select(x => new
{
Id = x.Interni.Id,
Condomino = x.Interni.Condomino,
Interno = x.Interni.Interno,
NomePiano = x.Interni.Piani.NomePiano,
NomeCondominio = x.Interni.Stabili.Nome,
x.MillesimiTabellaC?.millesimi_fabbisogno_acs,
x.MillesimiTabellaC?.millesimi_fabbisogno_riscaldamento
})
.ToList();

SUM and COUNT in single LINQ to SQL query

I'm trying to create the following query in LINQ-TO-SQL.
select count(*), sum( o.CostInCents ) from Orders o
where Flag = true;
I came up with the following LINQ query:
var q = db.Orders
.Where(o => o.Flag )
var result = q
.GroupBy(o => 1)
.Select(g => new MyDTO
{
NoOfOrders = g.Count(),
TotalInCents = g.Sum(o => o.CostInCents )
})
.SingleOrDefaultAsync();
Is there a better way?
Is .GroupBy(o => 1) even OK?
The other option would be to do two queries, like below.
var q = db.Orders
.Where(o => o.Flag );
//No groupBy
var result2 = new MyDTO
{
NoOfCostedOrders = q.Count(),//hit the db
TotalInCents = q.Sum(o => o.CostInCents )//hit the db 2nd time
};
How should I judge which approach is better?
Thanks in advance!

This query can be rewritten in sql format as follows
var orderList = db.Orders.Where(o => o.Flag );
var orderSummary = from o in orderList
group o by 1 into p
select new
{
Items = p.Count(),
Total = p.Sum( x => x.CostInCents)
}

I think what you are searching for is the following slight adjustment:
var q = db.Orders
.Where(o => o.Flag).Select(o => o.CostInCents).ToList(); // hit the db here once
//No groupBy
var result2 = new MyDTO
{
NoOfCostedOrders = q.Count(), // don't hit the db
TotalInCents = q.Sum() // don't hit the db a 2nd time
};
If you have a question to my adjustment feel free to comment.

How to find `Distinct` records in LINQ?

I want to find distinct records in entity framework. My code is as below
var distinctYear = _objCalRepos
.GetDetails()
.Select(o => new CalendarList { Mdate = o.Mdate.Year.ToString() })
.Distinct()
.ToList();
ddlYear.DataSource = distinctYear;
ddlYear.DataTextField = "Mdate";
ddlYear.DataValueField = "Mdate";
ddlYear.DataBind();
Here Distinct not works. It will return all entries(duplicated).

Distinct is not working, probably because CalendarList is not comparable.
Try this:
var distinctYear = _objCalRepos
.GetDetails()
.Select(o => o.Mdate.Year.ToString())
.Distinct()
.AsEnumerable()
.Select(o => new CalendarList { Mdate = o }))
.ToList();

You can use GroupBy
var distinctYear = _objCalRepos
.GetDetails()
.Select(o => new CalendarList { Mdate = o.Mdate.Year.ToString() })
.GroupBy(cl => cl.Mdate )
.Select(g => g.First())
.ToList();

C# Linq -Extension Method

How to use extension methods to form the second query as the first one.
1) var query = from cm in cust
group cm by cm.Customer into cmr
select (new { CKey = cmr.Key, Count = cmr.Count() });
(second query is not well formed)
2) var qry = cust.GroupBy(p => p.Customer).
Select(new { CKey = p.Key, Count = p.Count });

Try this:
var query = cust.GroupBy(p => p.Customer)
.Select(g => new { CKey = g.Key, Count = g.Count() });
You can also simplify this into a single call to this GroupBy overload though:
var query = cust.GroupBy(p => p.Customer,
(key, g) => new { CKey = key, Count = g.Count() });
Note that I've changed the name of the lambda expression's parameter name for the second line to g - I believe that gives more of a clue that you're really looking at a group rather than a single entity.
I've also moved the dot onto the second line in the form that still uses Select - I find this makes the query easier to read; I usually line up the dots, e.g.
var query = foo.Where(...)
.OrderBy(...)
.GroupBy(...)
.Select(...)

I think you need:
var qry = cust.GroupBy(p => p.Customer)
.Select(grp => new { CKey = grp.Key, Count = grp.Count() });