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Linq Query Join to Subquery with In clause

I have a query from an old application that I am attempting to convert to an Entity Framework Core application. The relationship I am working with is a simple one to many, where one Order can have many OrderEvents. The old query looks something like this:
select Order.Id, mostRecent.*
from Order
left join OrderEvent mostRecent
on mostRecent.Id in (select top(1) Id
from OrderEvent
where OrderEvent.OrdId = Order.Id
and OrderEvent.PostDateTime is not null
order by OrderEvent.PostDateTime desc)
where Order.SomeColumn = 'some value'
I am struggling with figuring out how to write this query in LINQ. It doesnt seem that one can use anything other than equals when doing using join, so I first attempted something like:
var test = (from order in _context.Ord.AsNoTracking()
join mostRecentQuery in _context.OrdEvt.AsNoTracking()
on (from orderEvent in _context.OrdEvt.AsNoTracking()
where orderEvent.PostDateTime != null && orderEvent.OrdId == order.Id
orderby orderEvent.PostDateTime descending
select orderEvent.Id).FirstOrDefault()
equals mostRecentQuery.Id
into mostRecentResults
from mostRecent in mostRecentResults.DefaultIfEmpty()
select new
{
OrderId = order.Id,
OrderEvent = mostRecent
}).ToList();
Whatever sql this spit out, it appears to be so slow that I cant even run it connected to Sql Server. I am however able to run this query when using Sqlite and it generates the following sql:
SELECT 'big list of fields....'
FROM "Ord" AS "order"
LEFT JOIN "OrdEvt" AS "mostRecentQuery" ON COALESCE((
SELECT "orderEvent0"."Id"
FROM "OrdEvt" AS "orderEvent0"
WHERE "orderEvent0"."PostDateTime" IS NOT NULL AND ("orderEvent0"."OrdId" = "order"."Id")
ORDER BY "orderEvent0"."PostDateTime" DESC
LIMIT 1
), X'00000000000000000000000000000000') = "mostRecentQuery"."Id"
This is close do what I am going for, but Im not sure why the COALESCE function is being used.
Is it possible to represent the query I am trying to convert, in Linq query syntax?

I'm not sure what your problem is, because you left out why it doesn't compile.
Bute here and simple example whith a join and choosing one event for an order:
public class Order
{
public int OrderId { get; set; }
public string OrderName { get; set; }
}
public class OrderEvent
{
public int OrderId { get; set; }
public string EventName { get; set; }
public DateTime Date { get; set; }
}
public static void Main(string[] args) // Here our Exe-Args are delivered to our program..
{
List<Order> orders = new List<Order>()
{
new Order() { OrderId = 1, OrderName = "One" }
};
List<OrderEvent> orderEvents = new List<OrderEvent>()
{
new OrderEvent() { OrderId = 1, EventName = "EventTwo", Date = DateTime.Now.AddHours(-1) },
new OrderEvent() { OrderId = 1, EventName = "EventOne", Date = DateTime.Now },
new OrderEvent() { OrderId = 1, EventName = "EventThree", Date = DateTime.Now.AddHours(-2) },
new OrderEvent() { OrderId = 2, EventName = "EventX", Date = DateTime.Now },
};
var tmp = orders.GroupJoin( // Join something to our orders
orderEvents, // join all events
o => o.OrderId, // key of our order
e => e.OrderId, // foreign-key to order in our event
(o, es) => // lambda-expression "selector". One order "o" and multiple events "es"
es.OrderByDescending(e => e.Date) // We order our events
.Select(s => new // we build a new object
{
OrderId = o.OrderId,
OrderName = o.OrderName,
RecentEvent = s.EventName
}).First()); // we choose the object with biggest date (first after orderDesc)
foreach (var item in tmp)
{
Console.WriteLine(item.OrderId + ", " + item.OrderName + ", " + item.RecentEvent);
}
tmp = from o in orders
join e in orderEvents on o.OrderId equals e.OrderId into es
select new
{
OrderId = o.OrderId,
OrderName = o.OrderName,
RecentEvent = (from x in es orderby x.Date descending select x.EventName).First()
};
foreach (var item in tmp)
{
Console.WriteLine(item.OrderId + ", " + item.OrderName + ", " + item.RecentEvent);
}
}

Calculating product's quantity with nested EF Query

I want to calculate the quantity of every product. I want to use these tables for this.
I want take result like this.
StockId | StockName | Sum(ProductNumber) [actually product quantity]
I tried this code for can take right result but It is returning null value even though it is the condition that satisfies the conditions.
CimriContext context = new CimriContext();
public ICollection<StockDto.StockHeader> FillDataGrid(int userCompanyId)
{
ICollection<StockDto.StockHeader> Stocks = context.Stocks.Where(c => c.UserCompanyId==userCompanyId).
Select(c => new StockDto.StockHeader()
{
StockId = c.StockId,
StockName = c.StockName,
Piece=0
}).ToList();
ICollection<StockDto.StockHeader> ProductHeader = new List<StockDto.StockHeader>();
foreach (var products in Stocks)
{
products.Piece = context.ProductTransactions.Where(p => p.StockId==products.StockId).Sum(s => s.ProductNumber);
ProductHeader.Add(products);
}
return ProductHeader.ToList();
}

You want to retrieve ProductCount for per stock record. You can perform it by joining two tables and grouping StockId and StockNumber like this;
var query = from stocks in context.Stocks
join products in context.ProductTransactions ON stocks.StockId equals products.StockId
where stocks.UserCompanyId = userCompanyId
group stocks by new
{
stocks.StockId,
stocks.StockName
} into gcs
select new
{
StockId = gcs.Key.StockId,
StockName = gcs.Key.StockName,
ProductCounts = gcs.Count()
};

linq combine 2 tables into one list

I have 2 tables in db - one is EmploymentRecords one is EmploymentVerificationRecords
I want to query both tables and return one List
I have a Model (simplified example):
Public Class Record
{
int ID {get; set;}
string name {get; set;}
bool IsVerification {get; set;}
}
I want to have some type of query in LINQ like :
var records = from a in _context.EmploymentRecords
join b in _context.EmploymentVerificationRecords on a.id equals b.id
where a.UserID = 1
select new Record() { .ID = a.id, .name = a.name, .IsVerification = false}
// I also want to select a new Record() for each b found
see - I also want a new Record() added to the results for each record found in the second table , for these results IsVerification would be True

You can select everything from DB as it is selected now (but I would rather use join/into to do that) and then flatten results into one big collection using LINQ to Objects.
Following should do the trick:
var records
= (from a in _context.EmploymentRecords
join b in _context.EmploymentVerificationRecords on a.id equals b.id into bc
where a.UserID = 1
select new {
a = new Record() { ID = a.id, name = a.name, IsVerification = false},
b = bc.Select(x => new Record() { ID = x.ID, name = b.name, IsVerification = true })
}).AsEnumerable()
.SelectMany(x => (new [] { x.a }).Concat(x.b));

Compare with list using linq

I have a list cids that I am fetching like this
var cids = _service.Employee.Where(i => i.EmpID == _empID).Select(j => j.ClientID).ToList();
I want to compare this list with Patient Entity and get all the records of patients that matches the clientId in the cid list
Patient Entity is like this
class Patient
{
Int PatientID{ get; set;}
Int ClientID{get; set;}
string PatientName{get; set;}
}
Right now I am doing it like this
foreach(var item in cids)
{
var pp = from p1 in _service.Patients
where p1.ClientId == item
select new PatientDTO
{
PatientID = p1.PatientID,
PatientName = p1.PatientName,
};
prec.Add(pp);
}
Is there a way to do it with Linq without using foreach

You can use Contains on your List (you don't need the ToList, by the way : this would avoid 2 queries on db).
var allPp = _service.Patients.Where(p1 => cids.Contains(p1.ClientId))
.Select(m >= new PatientDTO) {
PatientID = m.PatientID,
PatientName = m.PatientName
});
But the most performant way, in a db world, would be a join
from emp in _service.Employee.Where(i => i.EmpID == _empID)
join patient in _service.Patients on emp.ClientId equals patient.ClientId
select new PatientDTO {
PatientID = patient.PatientID,
PatientName = patient.PatientName,
}

Use Enumberable.Intersect to fetch common records.
var commonClients = cids.Intersect<int>(_service.Patients.Select(x => x.ClientID));
var person = _service.Patients.Where(x => commonClients.Contains(x.ClientID));

LINQ - Left join and Group by and Sum

I have two lists                                                      
var stores = new[]
{
new { Code = 1, Name = "Store 1" },
new { Code = 2, Name = "Store 2" }
};
var orders = new[]
{
new { Code = 1, StoreCode = 1, TotalValue = 14.12 },
new { Code = 2, StoreCode = 1, TotalValue = 24.12 }
};
OUTPUT
StoreName = Store 1 | TotalValue = 38.24
StoreName = Store 2 | TotalValue = 0
How can I translate this into LINQ to SQL?                                             
var lj = (from s in stores
join o in orders on s.Code equals o.StoreCode into joined
from j in joined.DefaultIfEmpty()
group s by new
{
StoreCode = s.Code,
StoreName = s.Name
}
into grp
select new
{
StoreName = grp.Key.StoreName,
TotalValue = ???
}).ToList();

When you doing group join, all orders related to store will be in group, and you will have access to store object. So, simply use s.Name to get name of store, and g.Sum() to calculate total of orders:
var lj = (from s in db.stores
join o in db.orders on s.Code equals o.StoreCode into g
select new {
StoreCode = s.Name,
TotalValue = g.Sum(x => x.TotalValue)
}).ToList();
Note - from your sample it looks like you don't need to group by store name and code, because code looks like primary key and it's unlikely you will have several stores with same primary key but different names.