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LINQ Left Join SQL equivalent via DefaultIfEmpty fails to return results

I have had an extensive look around on SE, tried all of the suggestions, checked out MSDN how to perform Left Join equivalent in LINQ to SQL and I have constructed my LINQ query according to MSDN example.
However, the result is not what SQL would return and I am completely lost as to where am I going wrong.
Here is some details:
I have two tables, Customers and Reports. A customer can submit many reports or none. In the current state I have many more reports than customers.
LINQ code:
var query = {from c in customers
join r in reports on c.Id equals r.Id into temp
from items in temp.DefaultIfEmpty()
select new {
c.Id,
LastReportDate = items?.DateCreated ?? DateTime.MinValue
}).ToList();
SQL code:
SELECT [Customers].[Id], R.LastReport AS LastReportDate FROM [Customers]
LEFT JOIN (
SELECT Reports.Id, MAX( [Reports].[Created] ) AS LastReport
FROM Reports GROUP BY Reports.Id
) AS r ON [Customers].[Id] = r.[Id]
The problem is that the query returns number of elements equal to number of reports. However, what I want is to get a list with all customers and for those who have submitted a report I wish to display the date of the most recent report, for those who have not submitted anything, I am happy to leave it NULL or DateTime.MinValue
Any help would be greatly appreciated. I guess I am missing a group by call somewhere in my LINQ code...

Im thinking probably something like this:
var query =
from c in customers
join r in reports on c.Id equals r.Id into g
select new
{
c.Id,
LastReportDate = g.Max(x => (DateTime?)x.Created)
};

you are now joining on join r in reports on c.Id equals r.Id into temp
this looks like: join on a customer.Id on Reports.Id, since you say there are 1 to many relation/rapport. I think your table will have a Reports.CustomerId. Is this correct?
So your query should something look like:
var results = customer.Where(c => c.Reports.Any())
.SelectMany(c => {c, c.Reports.Max(r => r.Created)})
.ToList();
the select comes out of my head, so i am probably missing something ;)
Have you tried LinqPad ? There you can type your linq-queries, and directly see your sql code and results. Works like a charm!

How to join a list and large lists/tables using LINQ

Initially I have such a list :
List<Car> cars = db.Car.Where(x => x.ProductionYear == 2005).ToList();
Then I'm trying to join this list with two large tables using LINQ like this :
var joinedList = (from car in cars
join driver in db.Driver.ToList()
on car.Id equals driver.CarId
join building in db.Building.ToList()
on driver.BuildingId equals building.Id
select new Building
{
Name = building.Name;
Id = building.Id;
City = building.City;
}).ToList();
Both Driver and Building tables have about 1 million rows. When I run this join I get out of memory exception. How can I make this join work? Should I make the join operation on database? If yes, how can I carry cars list to the db? Thanks in advance.

Even if you remove the .ToList() calls inside your join, you code will still pull all the data and perform the join in-memory and not in SQL server. This is because you're using a local list cars in your join. The below should solve your problem:
var joinedList = (from car in db.Car.Where(x => x.ProductionYear == 2005)
join driver in db.Driver
on car.Id equals driver.CarId
join building in db.Building
on driver.BuildingId equals building.Id
select new Building
{
Name = building.Name;
Id = building.Id;
City = building.City;
}).ToList();
You can remove the last .ToList() and do some paging if you expect to get too many records in the results.

even If You have removed .ToList() replace in .AsQueryable()
AsQueryable Faster then ToList And AsEnumerable
If you create an IQueryable, then the query may be converted to sql
and run on the database server
If you create an IEnumerable, then all rows will be pulled into
memory as objects before running the query.
In both cases if you don't call a ToList() or ToArray() then query
will be executed each time it is used, so, say, you have an
IQueryable and you fill 4 list boxes from it, then the query will be
run against the database 4 times.
so following Used Linq query
var joinedList = (from car in db.Car.Where(x => x.ProductionYear == 2005).AsQueryable()
join driver in db.Driver.AsQueryable()
on car.Id equals driver.CarId
join building in db.Building.AsQueryable()
on driver.BuildingId equals building.Id
select new Building
{
Name = building.Name,
Id = building.Id,
City = building.City,
}).ToList();

First don't ever try ToList() while using LINQ(you can) but make sure that you use ToList() as less as possible in a very rare scenarios only.
Every time you will get OutOfMemoryException when the table contains many rows.
So, here is the code for your question:
var joinedList = (from car in db.Car.GetQueryable().Where(x => x.ProductionYear == 2005)
join driver in db.Driver.GetQueryable() on car.Id equals driver.CarId
join building in db.Building.GetQueryable() on driver.BuildingId equals building.Id
select new Building
{
Name = building.Name;
Id = building.Id;
City = building.City;
}).ToList();

Using DISTINCT on a subquery to remove duplicates in Entity Framework

I have question about use of Distinct with Entity Framework, using Sql 2005. In this example:
practitioners = from p in context.Practitioners
join pn in context.ProviderNetworks on
p.ProviderId equals pn.ProviderId
(notNetworkIds.Contains(pn.Network))
select p;
practitioners = practitioners
.Distinct()
.OrderByDescending(p => p.UpdateDate);
data = practitioners.Skip(PageSize * (pageOffset ?? 0)).Take(PageSize).ToList();
It all works fine, but the use of distinct is very inefficient. Larger result sets incur unacceptable performance. The DISTINCT is killing me. The distinct is only needed because multiple networks can be queried, causing Providers records to be duplicated. In effect I need to ask the DB "only return providers ONCE even if they're in multiple networks". If I could place the DISTINCT on the ProviderNetworks, the query runs much faster.
How can I cause EF to add the DISTINCT only the subquery, not to the entire resultset?
The resulting simplified sql I DON'T want is:
select DISTINCT p.* from Providers
inner join Networks pn on p.ProviderId = pn.ProviderId
where NetworkName in ('abc','def')
IDEAL sql is:
select p.* from Providers
inner join (select DISTINCT ProviderId from Networks
where NetworkName in ('abc','def'))
as pn on p.ProviderId = pn.ProviderId
Thanks
Dave

I dont think you need a Distinct here but a Exists (or Any as it is called in Linq)
Try this:
var q = (from p in context.Practitioners
where context.ProviderNetworks.Any(pn => pn.ProviderId == p.ProviderId && notNetworkIds.Contains(pn.Network))
orderby p.UpdateDate descending
select p).Skip(PageSize * (pageOffset ?? 0)).Take(PageSize).ToList();

Is this LINQ Query "correct"?

I have the following LINQ query, that is returning the results that I expect, but it does not "feel" right.
Basically it is a left join. I need ALL records from the UserProfile table.
Then the LastWinnerDate is a single record from the winner table (possible multiple records) indicating the DateTime the last record was entered in that table for the user.
WinnerCount is the number of records for the user in the winner table (possible multiple records).
Video1 is basically a bool indicating there is, or is not a record for the user in the winner table matching on a third table Objective (should be 1 or 0 rows).
Quiz1 is same as Video 1 matching another record from Objective Table (should be 1 or 0 rows).
Video and Quiz is repeated 12 times because it is for a report to be displayed to a user listing all user records and indicate if they have met the objectives.
var objectiveIds = new List<int>();
objectiveIds.AddRange(GetObjectiveIds(objectiveName, false));
var q =
from up in MetaData.UserProfile
select new RankingDTO
{
UserId = up.UserID,
FirstName = up.FirstName,
LastName = up.LastName,
LastWinnerDate = (
from winner in MetaData.Winner
where objectiveIds.Contains(winner.ObjectiveID)
where winner.Active
where winner.UserID == up.UserID
orderby winner.CreatedOn descending
select winner.CreatedOn).First(),
WinnerCount = (
from winner in MetaData.Winner
where objectiveIds.Contains(winner.ObjectiveID)
where winner.Active
where winner.UserID == up.UserID
orderby winner.CreatedOn descending
select winner).Count(),
Video1 = (
from winner in MetaData.Winner
join o in MetaData.Objective on winner.ObjectiveID equals o.ObjectiveID
where o.ObjectiveNm == Constants.Promotions.SecVideo1
where winner.Active
where winner.UserID == up.UserID
select winner).Count(),
Quiz1 = (
from winner2 in MetaData.Winner
join o2 in MetaData.Objective on winner2.ObjectiveID equals o2.ObjectiveID
where o2.ObjectiveNm == Constants.Promotions.SecQuiz1
where winner2.Active
where winner2.UserID == up.UserID
select winner2).Count(),
};

You're repeating join winners table part several times. In order to avoid it you can break it into several consequent Selects. So instead of having one huge select, you can make two selects with lesser code. In your example I would first of all select winner2 variable before selecting other result properties:
var q1 =
from up in MetaData.UserProfile
select new {up,
winners = from winner in MetaData.Winner
where winner.Active
where winner.UserID == up.UserID
select winner};
var q = from upWinnerPair in q1
select new RankingDTO
{
UserId = upWinnerPair.up.UserID,
FirstName = upWinnerPair.up.FirstName,
LastName = upWinnerPair.up.LastName,
LastWinnerDate = /* Here you will have more simple and less repeatable code
using winners collection from "upWinnerPair.winners"*/

The query itself is pretty simple: just a main outer query and a series of subselects to retrieve actual column data. While it's not the most efficient means of querying the data you're after (joins and using windowing functions will likely get you better performance), it's the only real way to represent that query using either the query or expression syntax (windowing functions in SQL have no mapping in LINQ or the LINQ-supporting extension methods).
Note that you aren't doing any actual outer joins (left or right) in your code; you're creating subqueries to retrieve the column data. It might be worth looking at the actual SQL being generated by your query. You don't specify which ORM you're using (which would determine how to examine it client-side) or which database you're using (which would determine how to examine it server-side).
If you're using the ADO.NET Entity Framework, you can cast your query to an ObjectQuery and call ToTraceString().
If you're using SQL Server, you can use SQL Server Profiler (assuming you have access to it) to view the SQL being executed, or you can run a trace manually to do the same thing.
To perform an outer join in LINQ query syntax, do this:
Assuming we have two sources alpha and beta, each having a common Id property, you can select from alpha and perform a left join on beta in this way:
from a in alpha
join btemp in beta on a.Id equals btemp.Id into bleft
from b in bleft.DefaultIfEmpty()
select new { IdA = a.Id, IdB = b.Id }
Admittedly, the syntax is a little oblique. Nonetheless, it works and will be translated into something like this in SQL:
select
a.Id as IdA,
b.Id as Idb
from alpha a
left join beta b on a.Id = b.Id

It looks fine to me, though I could see why the multiple sub-queries could trigger inefficiency worries in the eyes of a coder.
Take a look at what SQL is produced though (I'm guessing you're running this against a database source from your saying "table" above), before you start worrying about that. The query providers can be pretty good at producing nice efficient SQL that in turn produces a good underlying database query, and if that's happening, then happy days (it will also give you another view on being sure of the correctness).

Trouble converting a bit of TSQL to LINQ to Entities

Sorry about the vague title, not sure what verbage I should be using. I have a query similar to this (re-worked to save space):
SELECT
*
FROM
Publishers p
INNER JOIN Authors a
ON p.AuthorID = a.AuthorID
INNER JOIN Books b
ON a.BookID = b.BookID
WHERE
p.PublisherName = 'Foo'
ORDER BY
b.PublicationDate DESC
I tried to re-write it as such:
var query =
from publisher in ctx.Publishers
from author in publisher.Authors
from books in author.Books
...
but got the following error:
Error 1 An expression of type 'Models.Books' is not allowed in a
subsequent from clause in a query expression with source type
'System.Linq.IQueryable<AnonymousType#1>'. Type inference failed in the
call to 'SelectMany'.
I can re-write the LINQ to make it work by just joining the tables, as I would in SQL, but I thought I could accomplish what I want to do by their relationships - I'm just a bit confused why I can get publisher.Authors, but not author.Books.

Check that you have a relationship in your DB from Authors to Books.

Try this...
var result = (from pItem in ctx.Publishers
join aItem in ctx.Authors on pItem.AuthorId equals aItem.AuthorId
join bItem in ctx.Books on pItem.BookId equals bItem.BookId
where pItem.PublisherName== "Foo"
select new {
// Fields you want to select
}
).ToList();
i don't know exact relationship of the tables but you can an idea from this one.