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How to write linq query for this sql statement

How would you write a linq query with the following SQL statement. I've tried several methods referenced on stackoverflow but they either don't work with the EF version I'm using (EF core 3.5.1) or the DBMS (SQL Server).
select a.ProductID, a.DateTimeStamp, a.LastPrice
from Products a
where a.DateTimeStamp = (select max(DateTimeStamp) from Products where a.ProductID = ProductID)
For reference, a couple that I've tried (both get run-time errors).
var results = _context.Products
.GroupBy(s => s.ProductID)
.Select(s => s.OrderByDescending(x => x.DateTimeStamp).FirstOrDefault());
var results = _context.Products
.GroupBy(x => new { x.ProductID, x.DateTimeStamp })
.SelectMany(y => y.OrderByDescending(z => z.DateTimeStamp).Take(1))
Thanks!

I understand you would like to have a list of the latest prices of each products?
First of all I prefer to use group by option even over 1st query
select a.ProductID, a.DateTimeStamp, a.LastPrice
from Products a
where a.DateTimeStamp IN (select max(DateTimeStamp) from Products group by ProductID)
Later Linq:
var maxDateTimeStamps = _context.Products
.GroupBy(s => s.ProductID)
.Select(s => s.Max(x => x.DateTimeStamp)).ToArray();
var results = _context.Products.Where(s=>maxDateTimeStamps.Contains(s.DateTimeStamp));
-- all assuming that max datetime stamps are unique

I've managed to do it with the following which replicates the correlated sub query in the original post (other than using TOP and order by instead of the Max aggregate), though I feel like there must be a more elegant way to do this.
var results = from x
in _context.Products
where x.DateTimeStamp == (from y
in _context.Products
where y.ProductID == x.ProductID
orderby y.DateTimeStamp descending
select y.DateTimeStamp
).FirstOrDefault()
select x;

I prefer to break up these queries into IQueryable parts, do you can debug each "step".
Something like this:
IQueryable<ProductOrmEntity> pocoPerParentMaxUpdateDates =
entityDbContext.Products
//.Where(itm => itm.x == 1)/*if you need where */
.GroupBy(i => i.ProductID)
.Select(g => new ProductOrmEntity
{
ProductID = g.Key,
DateTimeStamp = g.Max(row => row.DateTimeStamp)
});
//// next line for debugging..do not leave in for production code
var temppocoPerParentMaxUpdateDates = pocoPerParentMaxUpdateDates.ToListAsync(CancellationToken.None);
IQueryable<ProductOrmEntity> filteredChildren =
from itm
in entityDbContext.Products
join pocoMaxUpdateDatePerParent in pocoPerParentMaxUpdateDates
on new { a = itm.DateTimeStamp, b = itm.ProductID }
equals
new { a = pocoMaxUpdateDatePerParent.DateTimeStamp, b = pocoMaxUpdateDatePerParent.ProductID }
// where
;
IEnumerable<ProductOrmEntity> hereIsWhatIWantItems = filteredChildren.ToListAsync(CancellationToken.None);
That last step, I am putting in an anonymous object. You can put the data in a "new ProductOrmEntity() { ProductID = pocoMaxUpdateDatePerParent.ProductID }...or you can get the FULL ProductOrmEntity object. Your original code, I don't know if getting all columns of the Product object is what you want, or only some of the columns of the object.

How to sort something in LINQ based on many dates?

Hello this is a LINQ Query but it doesn't sort properly because four different dates are involved.
var EventReportRemarks = (from i in _context.pm_main_repz
.Include(a => a.PM_Evt_Cat)
.Include(b => b.department)
.Include(c => c.employees)
.Include(d => d.provncs)
where i.department.DepartmentName == "Finance"
orderby i.English_seen_by_executive_on descending
orderby i.Brief_seen_by_executive_on descending
orderby i.French_seen_by_executive_on descending
orderby i.Russian_seen_by_executive_on descending
select i).ToList();
All i want is that it should somehow combine the four dates and sort them in group not one by one.
For Example, at the moment it sorts all English Reports based on the date that executive has seen it, then Brief Report and So on.
But i want that it should check which one is seen first and so on. For example if the first report which is seen is French, then Brief, then English then Russian, so it should sort it accordingly.
Is it Possible??

You need to have them all in one column. The approach I would do, assuming that the value of the respective cells is null, when you don't want them to show up in the order by:
var EventReportRemarks = (from i in _context.pm_main_repz
.Include(a => a.PM_Evt_Cat)
.Include(b => b.department)
.Include(c => c.employees)
.Include(d => d.provncs)
where i.department.DepartmentName == "Finance"
select new
{
Date =
(
i.English_seen_by_executive_on != null ? i.English_seen_by_executive_on :
i.Brief_seen_by_executive_on != null ? i.Brief_seen_by_executive_on :
i.French_seen_by_executive_on != null ? i.French_seen_by_executive_on :
i.Russian_seen_by_executive_on
)
}).ToList().OrderBy(a => a.Date);
In the select clause you could add more columns if you whish.
Reference taken from here.

Why not just use .Min() or .Max() on the dates and then .OrderBy() or .OrderByDescending() based on that?
Logic is creating a new Enumerable (here, an array) with the 4 dates for the current line, and calculate the Max/Min of the 4 dates: this results in getting the latest/earliest of the 4. Then order the records based on this value.
var EventReportRemarks = (from i in _context.pm_main_repz
.Include(a => a.PM_Evt_Cat)
.Include(b => b.department)
.Include(c => c.employees)
.Include(d => d.provncs)
where i.department.DepartmentName == "Finance"
select i)
.OrderBy(i => new[]{
i.English_seen_by_executive_on,
i.Brief_seen_by_executive_on,
i.French_seen_by_executive_on,
i.Russian_seen_by_executive_on
}.Max())
.ToList();

Your problem is not a problem if you use method syntax for your LINQ query instead of query syntax.
var EventReportRemarks = _context.pm_main_repz
.Where(rep => rep.Department.DepartmentName == "Finance")
.OrderByDescending(rep => rep.English_seen_by_executive_on)
.ThenByDescending(rep => rep.Brief_seen_by_executive_on)
.ThenByDescending(rep => rep.French_seen_by_executive_on descending)
.ThenByDescending(rep => resp.Russian_seen_by_executive_on descending)
.Select(rep => ...);
Optimization
One of the slower parts of a database query is the transport of selected data from the DBMS to your local process. Hence it is wise to limit the transported data to values you actually plan to use.
You transport way more data than you need to.
For example. Every pm_main_repz (my, you do love to use easy identifiers for your items, don't you?), every pm_main_repz has zero or more Employees. Every Employees belongs to exactly one pm_main_repz using a foreign key like pm_main_repzId.
If you use include to transport pm_main_repz 4 with his 1000 Employees every Employee will have a pm_main_repzId with value 4. You'll transport this value 1001 times, while 1 time would have been enough
Always use Select to select data from the database and Select only the properties you actually plan to use. Only use Include if you plan to update the fetched objects
Consider using a proper Select where you only select the items that you actually plan to use:
.Select(rep => new
{
// only Select the rep properties you actually plan to use:
Id = rep.Id,
Name = rep.Name,
...
Employees = rep.Employees.Select(employee => new
{
// again: select only the properties you plan to use
Id = employee.Id,
Name = employee.Name,
// not needed: foreign key to pm_main_repz
// pm_main_repzId = rep.pm_main_repzId,
})
.ToList(),
Department = new
{
Id = rep.Department,
...
}
// etc for pm_evt_cat and provencs
});

EF Core 2.0 Group By other properties

I have 2 tables:
USERS
UserId
Name
Scores (collection of table Scores)
SCORES
UserId
CategoryId
Points
I need to show all the users and a SUM of their points, but also I need to show the name of the user. It can be filtered by CategoryId or not.
Context.Scores
.Where(p => p.CategoryId == categoryId) * OPTIONAL
.GroupBy(p => p.UserId)
.Select(p => new
{
UserId = p.Key,
Points = p.Sum(s => s.Points),
Name = p.Select(s => s.User.Name).FirstOrDefault()
}).OrderBy(p => p.Points).ToList();
The problem is that when I add the
Name = p.Select(s => s.User.Name).FirstOrDefault()
It takes so long. I don't know how to access the properties that are not inside the GroupBy or are a SUM. This example is very simple becaouse I don't have only the Name, but also other properties from User table.
How can I solve this?

It takes so long because the query is causing client evaluation. See Client evaluation performance issues and how to use Client evaluation logging to identify related issues.
If you are really on EF Core 2.0, there is nothing you can do than upgrading to v2.1 which contains improved LINQ GroupBy translation. Even with it the solution is not straight forward - the query still uses client evaluation. But it could be rewritten by separating the GroupBy part into subquery and joining it to the Users table to get the additional information needed.
Something like this:
var scores = db.Scores.AsQueryable();
// Optional
// scores = scores.Where(p => p.CategoryId == categoryId);
var points = scores
.GroupBy(s => s.UserId)
.Select(g => new
{
UserId = g.Key,
Points = g.Sum(s => s.Points),
});
var result = db.Users
.Join(points, u => u.UserId, p => p.UserId, (u, p) => new
{
u.UserId,
u.Name,
p.Points
})
.OrderBy(p => p.Points)
.ToList();
This still produces a warning
The LINQ expression 'orderby [p].Points asc' could not be translated and will be evaluated locally.
but at least the query is translated and executes as single SQL:
SELECT [t].[UserId], [t].[Points], [u].[UserId] AS [UserId0], [u].[Name]
FROM [Users] AS [u]
INNER JOIN (
SELECT [s].[UserId], SUM([s].[Points]) AS [Points]
FROM [Scores] AS [s]
GROUP BY [s].[UserId]
) AS [t] ON [u].[UserId] = [t].[UserId]

Ordering not working in Entity Framework query

I'm currently battling a linq query for my application using Entity Framework (6.1.3)
The query is as follows:
var productPeriods = (from pp in ctx.ProductPeriods
where pp.IsActive && pp.Product.IsBuyBackForProduct == null && !pp.Product.ProductAddOns.Any() && pp.PowerRegionID == powerRegionId
select new
{
ProductPeriod = pp,
Price = pp.Prices
.OrderByDescending(x => x.Created)
.GroupBy(x => x.FirmID)
.Select(pr => pr.FirstOrDefault())
.OrderByDescending(x => x.ProductPrice)
.FirstOrDefault()
}).ToList();
The purpose of the query is to find the latest price from the prices collection of a product period, grouped by the firm ID and then select the best price of the latest prices from each firm.
This works perfectly in Linqpad, but the first OrderByDescending(x => x.Created) doesn't work when used in context of Entity Framework.
Does anyone knows why? And perhaps have a solution for it? :-)
Thanks in advance!
Update
Thanks for all replies. I've tried the following:
select new {
ProductPeriod = p,
Price = p.Prices.GroupBy(x => x.FirmID).Select(pr => pr.OrderByDescending(x => x.Created).ThenByDescending(x => x.ProductPrice).FirstOrDefault())
}
But it seems like ThenByDescending(x => x.ProductPrice) gets ignored as well. The prices are not sorted correctly in the output. They're output like this:
Price: 0,22940, Created: 06-03-2015 10:15:09,
Price: 0,23150, Created: 06-03-2015 10:05:48
Price: 0,20040, Created: 06-03-2015 09:24:24
Update 2 (solution for now)
I came to the solution that the initial query just returns the latest prices from each firm. There's currently three firms, so the performance should be alright.
Later in my code, where I'm actually using the latest and best price, I simply do an .OrderByDescending(x => x.ProductPrice).FirstOrDefault() and check if it's not null.
I.e:
var productPeriods = (from pp in ctx.ProductPeriods
where pp.IsActive && pp.Product.IsBuyBackForProduct == null && !pp.Product.ProductAddOns.Any() && pp.PowerRegionID == powerRegionId
select new
{
ProductPeriod = pp,
Prices = pp.Prices.GroupBy(x => x.FirmID).Select(pr => pr.OrderByDescending(x => x.Created).FirstOrDefault())
}).ToList();
Later in my code:
var bestPriceOfToday = period.Prices.OrderByDescending(x => x.ProductPrice).FirstOrDefault()

The problem is the commands you are using. OrderBy and OrderByDescending do NOT add additional order by statements to the resulting query but instead they CREATE the order by statement and eliminate all orderby statements that existed before.
In order to use multiple orderby's you need to do the following:
OrderBy or OrderByDescending
ThenBy or ThenByDescending
the ThenBy statements can be used 1 or more times they just add additional order statements to the resulting query.

According to yours update, omnit select and type:
select new {
ProductPeriod = p,
Price = p.Prices.GroupBy(x => x.FirmID)
.OrderByDescending(x =>x.Created).ThenByDescending(x=>x.ProductPrice).FirstOrDefault()
}
That select was useless and could be the cause of problem

How to select associate Table from many to many relationship

i have a many to many relationship tables, movie and tag in code first approach in my MVC application. i want the user to be able to select all movies that is associted with tag. for example i want to select all movies that has tag id =2. i need lambda query that will allow me do this.Any help with appreciated.

This is one way:
var moviesWithTag2 = context.Movies
.Where(m => m.Tags.Any(t => t.Id == 2))
.ToList();
This will return an empty collection if there is no tag with Id=2.
Alternative (which will return null if there is no tag with Id=2):
var moviesWithTag2 = context.Tags
.Where(t => t.Id == 2)
.Select(t => t.Movies)
.SingleOrDefault();
If you also want the Tag entity you could just load it including its movies:
var tag2 = context.Tags.Include(t => t.Movies)
.Single(t => t.Id == 2);
tag2.Movies will contain the movies with tag Id=2.