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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.

Converting SQL to LINQ with Multiple Tables and Group

There seem to be lots of questions about SQL to LINQ, but I can't seem to find examples with joined tables and grouping; specifically with a need to get data from multiple tables.
Take this simple SQL:
SELECT
s.showId, s.showName, v.venueName, Min(dateTime) startDate
FROM
shows s
INNER JOIN venues v ON s.venueId = v.venueId
INNER JOIN showDates d ON s.showId = d.showId
GROUP BY
s.showId
The best I can come up with is the following
var ungrouped = (
from s in db.Shows
join v in db.Venues on s.VenueId equals v.VenueId
join d in db.ShowDates on s.ShowId equals d.ShowId
select new { s, v, d }
).ToList();
var grouped = (
from s in ungrouped
group s by s.s.ShowId into grp
select new
{
showId = grp.Key,
name = (from g in grp select g.s.showName).FirstOrDefault(),
venue = (from g in grp select g.v.VenueName).FirstOrDefault(),
startDate = grp.Max(g => g.d.DateTime)
}
);
This works but it feels messy. I don't like:
It being split into two statements
Having to repeatedly write (from g in grp select ...).FirstOrDefault()
Bits like s.s.ShowId
How its vastly more lines of code than the SQL
This example is a simple one, it only gets worse when I have 5+ tables to join and 10+ columns to select.
Question: Is this the best way to do this, and I should just accept it; or is there a better way to write this query?

I am not sure if you are looking for something like this but it's a bit cleaner, it's not split in 2 statements and you might find it helpful. I couldn't use a dbcontext so I used lists to make sure the syntax is correct.
var res = Shows.Join(Venues,
show => show.VenueID,
venue => venue.VenueID,
(show, venue) => new { show, venue })
.Join(ShowDates,
val => val.show.ShowID,
showdate => showdate.ShowID,
(val, showDate) => new { val.show, val.venue, showDates = showDate })
.GroupBy(u => u.show.ShowID)
.Select(grp => new
{
showId = grp.Key,
name = grp.FirstOrDefault()?.show.showName,
venue = grp.FirstOrDefault()?.venue.VenueName,
startDate = grp.Max(g => g.showDates.DateTime)
});

we need to now realation beetwen them one to one or one to many , but not too far from this answer.
var GrouppedResult = Shows.Include(x=>x.Veneu).Include(x=>x.ShowDates)
.Where(x=>x.Veneu.Any()&&x.ShowDates.Any())
.GroupBy(x=>x.ShowId)
.Select(x=>///anything you want);
or
from show in Shows
join veneu in Veneu on veneu.VeneuId equals show.VeneuId
join showDates in ShowDates on showDates.ShowId=show.ShowID
group show by show.Id into grouppedShows
select new { ///what you want };

Linq using join and grouping

I'm trying to do something very simple.
I have two tables in my database that I would like to query using linq.
Table of Books, and table of GenreTypes. The result of this query would go to my web Api.
Here is a code snippet:
public List<BooksChart> GetBooksChart()
{
var results = from b in _dbcontext.Books
join g in _dbcontext.GenreTypes
on b.GenreTypeId equals g.Id
group g by g.Name into n
select (z => new BooksChart
{
category_name = n.Key,
value = n.Count()
}).ToList();
return results;
}
public class BooksChart
{
public string category_name;
public int value;
}
The results of the grouping "n" I would like to store them in BooksChart class to construct the Api.
This code is not compiling.
Previously, I was querying only one table of Books which I have divided into Books and GenreTypes.
My previous working code for querying Books was :
var results = _dbcontext
.Books
.GroupBy(x => x.GenreType)
.Select(z => new BooksPieChart
{
category_name = z.Key,
value = z.Count()
}).ToList();
return results;
EDIT
What I want to achieve in SQL is the following:
select count(*), g.Name
from books b, GenreTypes g
where b.GenreTypeId = g.Id
group by g.Name;

You are mixing the two syntax options of query and method. For query syntax you need to do the projection (select) like this:
return (from b in _dbcontext.Books
join g in _dbcontext.GenreTypes on b.GenreTypeId equals g.Id
group g by g.Name into n
select new BooksChart {
category_name = n.Key,
value = n.Count()
}).ToList();
The format of (z =>....) is the declaration of the labmda passed to the Select method.
Site notes:
As #Rabbi commented, since you are using EF, consider properly defining navigation properties. It will make querying simpler.
Side note for the sql - consider using joins instead of multiple tables in the from: INNER JOIN ON vs WHERE clause

The parentheses must surround the whole query, like so:
var results = (from b in _dbcontext.Books
join g in _dbcontext.GenreTypes
on b.GenreTypeId equals g.Id
group g by g.Name into n
select new BooksChart
{
category_name = n.Key,
value = n.Count()
}).ToList();
The compilation error is due to this (z => which is not needed at all.

Convert SQL query with multiple GroupBy columns to LINQ

SELECT
[TimeStampDate]
,[User]
,count(*) as [Usage]
FROM [EFDP_Dev].[Admin].[AuditLog]
WHERE [target] = '995fc819-954a-49af-b056-387e11a8875d'
GROUP BY [Target], [User] ,[TimeStampDate]
ORDER BY [Target]
My database table has the columns User, TimeStampDate, and Target (which is a GUID).
I want to retrieve all items for each date for each user and display count of entries.
The above SQL query works. How can I convert it into LINQ to SQL? Am using EF 6.1 and my entity class in C# has all the above columns.
Create Filter basically returns an IQueryable of the entire AuditLogSet :
using (var filter = auditLogRepository.CreateFilter())
{
var query = filter.All
.Where(it => it.Target == '995fc819-954a-49af-b056-387e11a8875d')
.GroupBy(i => i.Target, i => i.User, i => i.TimeStamp);
audits = query.ToList();
}
Am not being allowed to group by on 3 columns in LINQ and I am also not sure how to select like the above SQL query with count. Fairly new to LINQ.

You need to specify the group by columns in an anonymous type like this:-
var query = filter.All
.Where(it => it.Target == '995fc819-954a-49af-b056-387e11a8875d')
.GroupBy(x => new { x.User, x.TimeStampDate })
.Select(x => new
{
TimeStampDate= x.Key.TimeStampDate,
User = x.Key.User,
Usage = x.Count()
}).ToList();

Many people find query syntax simpler and easier to read (this might not be the case, I don't know), here's the query syntax version anyway.
var res=(from it in filter.All
where it.Target=="995fc819-954a-49af-b056-387e11a8875d"
group it by new {it.Target, it.User, it.TimeStampDate} into g
orderby g.Key.Target
select new
{
TimeStampDate= g.Key.TimeStampDate,
User=g.Key.User,
Usage=g.Count()
});
EDIT: By the way you don't need to group by Target neither OrderBy, since is already filtered, I'm leaving the exact translation of the query though.

To use GroupBy you need to create an anonymous object like this:
filter.All
.Where(it => it.Target == '995fc819-954a-49af-b056-387e11a8875d')
.GroupBy(i => new { i.Target, i.User, i.TimeStamp });

It is unnecessary to group by target in your original SQL.
filter.All.Where( d => d.Target == "995fc819-954a-49af-b056-387e11a8875d")
.GroupBy(d => new {d.User ,d.TimeStampDate} )
.Select(d => new {
User = d.Key.User,
TimeStampDate = d.Key.TimeStampDate,
Usage = d.Count()
} );

LINQ, Joins, Grouping and Aggregates

Using either a Join or GroupJoin, is there any way to produce aggregates values for fields in both the parent and child tables. Given an Orders table and an OrderDetails table, Using the 2 steps below I can obtain an aggregate (MAX) from the Orders and an aggregate (SUM) from the OrderDetails.
STEP 1:
var query = from o in orders
join d in details on o.OrderId equals d.OrderId
select new
{
order = o.OrderId,
maximum = o.UserId,
quantity = d.Quantity
};
Step 2:
var result = (from q in query
group q by q.order into g
select new
{
OrderId = g.Key,
MaxUnits = g.Max(q => q.maximum),
Available = (g.Max(q => q.maximum) - g.Sum(q => q.quantity))
});
However, when I try to combine these as in:
var finalresult = orders
.GroupJoin( details,
o => o.OrderId,
d => d.OrderDetailId,
(o, grp) => new {
OrderId = o.OrderId,
MaxUnits = grp.Max(o => o.maximum),
Available = (grp.Max(o => o.maximum) - grp.Sum(d => d.Quantity))
});
.. the value 'o' is out of scope inside the grouped set 'grp'. So grp.Max(o => o.maximum) results in an error. It appears that only aggregate values for the child table (OrderDetail) are available.
So does anyone know if it is possible to obtain aggregates from both the Child and Parent tables in a single query?

result is a single query. The beauty of LINQ and deferred execution is that no actual computation has happened in Step 1, only a query has been defined. Step 2 then builds ontop of that query to create another single query. When you execute result that query will be executed as a single block.
I recommend splitting up larger queries into smaller easier to understand pieces like in the first two examples. Using good names for the queries can make them much easier to read. For example, I might name query orderQuantities. from q in query does not convey much meaning, but from oq in orderQuantities lets me know what kind of data the query is over.
If you really think you need them together:
var query = orders.Join(details, o => o.OrderId, d => d.OrderId,
(o, d) => new {
order = o.OrderId,
maximum = o.UserId,
quantity = d.Quantity
}).GroupBy(oq => oq.order)
.Select(g => new {
OrderId = g.Key,
MaxUnits = g.Max(q => q.maximum),
Available = (g.Max(q => q.maximum) - g.Sum(q => q.quantity))
});
Now that is ugly...