We Keep Coding

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.

LINQ: Select the Min and Max values from a collection on an entity after grouping

The goal is to get the first DateTime and Last DateTime from a collection on an Entity (Foreign Key). My Entity is an organization and my collection are Invoices. I'm grouping results since Organizations unfortunately are not Unique. I'm dealing with duplicate data and cannot assume my organizations are unique so I'm grouping by a Number field on my Entity.
I'm using .NET Core 2.1.2 with Entity Framework.
I'm trying to get the following query generated from LINQ:
SELECT MIN([organization].[Id]) AS Id, MIN([organization].[Name]) AS Name,
MIN([organization].[Number]) AS Number, MIN([invoice].[Date])
AS First, MAX([invoice].[Date]) AS Last
FROM [organization]
INNER JOIN [invoice] ON [invoice].[OrganizationId] = [organization].[Id]
GROUP BY [organization].[Number], [organization].[Name]
ORDER BY [organization].[Name]
However I have no idea how to get to write the LINQ query to get it to generate this result.
I got as far as:
await _context
.Organization
.Where(z => z.Invoices.Any())
.GroupBy(organization => new
{
organization.Number,
organization.Name
})
.Select(grouping => new
{
Id = grouping.Min(organization => organization.Id),
Name = grouping.Min(organization => organization.Name),
Number= grouping.Min(organization => organization.Number),
//First = ?,
//Last = ?
})
.OrderBy(z => z.Name)
.ToListAsync();
I have no clue how to write the LINQ query in such a way that it generates the above.
I have a couple questions still:
Are the Min statements for Id, Name and Number correct ways of getting the
first element in the grouping?
Do I need a join statement or is "WHERE EXISTS" better (this got generated before I changed the code)?
Does anyone know how to finish writing the LINQ statement? Because I have to get the first and last Date from the Invoices Collection on my Organization Entity:
organization.Invoices.Min(invoice => invoice.Date)
organization.Invoices.Max(invoice => invoice.Date)

Here is the trick.
To make inner join by using collection navigation property simple use SelectMany and project all primitive properties that you need later (this is important for the current EF Core query translator). Then perform the GroupBy and project the key properties / aggregates. Finally do the ordering.
So
var query = _context
.Organization
.SelectMany(organization => organization.Invoices, (organization, invoice) => new
{
organization.Id,
organization.Number,
organization.Name,
invoice.Date
})
.GroupBy(e => new
{
e.Number,
e.Name
})
.Select(g => new
{
Id = g.Min(e => e.Id),
Name = g.Key.Name,
Number = g.Key.Number,
First = g.Min(e => e.Date),
Last = g.Max(e => e.Date),
})
.OrderBy(e => e.Name);
is translated to
SELECT MIN([organization].[Id]) AS [Id], [organization].[Name], [organization].[Number],
MIN([organization.Invoice].[Date]) AS [First], MAX([organization.Invoice].[Date]) AS [Last]
FROM [Organization] AS [organization]
INNER JOIN [Invoice] AS [organization.Invoice] ON [organization].[Id] = [organization.Invoice].[OrganizationId]
GROUP BY [organization].[Number], [organization].[Name]
ORDER BY [organization].[Name]

Order by user and then select max date

I have this LINQ query:
ArrayList arr = new ArrayList();
var data = conn.SCOT_DADOS.OrderByDescending(x => x.DATE)
.GroupBy(r => r.USER)
.ToList();
foreach (var item in data)
{
var itemdata = item.Where(r => r.DATE == item.Max(s => s.DATE));
var name = svc.GetUserName(itemdata.Select(r => r.USER).First().ToString());
var value = itemdata.Select(r => r.VALUE).First();
var date = itemdata.Select(r => r.DATE).First().ToString("dd/MM/yyyy HH:mm:ss");
arr.Add( new{ NAME = name, DATE = date, VALUE = value} );
}
This code will give me the latest result by DATE for each USER.
But the LINQ query is selecting all data from the user and then I'm getting the latest one in the foreach loop.
Is there any way to get only the last data in the LINQ query, so I don't have to take all the user data every time?
I have tried this:
var data = conn.SCOT_DADOS.OrderByDescending(x => x.DATE)
.GroupBy(r => r.USER)
.First()
.ToList();
And then treated item as an object, instead of running selects on it.
It gave me all the data for an individual user, which isn't what I want.
What can be done?
Edit 1:
I get this error if I try to swap OrderByDescending and GroupBy:
Error CS1061 'IGrouping' does not contain a
definition for 'DATE' and no extension method 'DATE' accepting a first
argument of type 'IGrouping' could be found (are
you missing a using directive or an assembly reference?)
Edit 2:
This is some sample data (the column names are not the same because I translated them for the question):
From the data presented, I'd have the results:

If the combination of the (USER, DATE) pair is unique (which seems to be the case when looking at the sample data), the requirement can be trimmed down to
return each record if there is no other record with the same USER and later DATE
which could be translated to the following LINQ query:
var result = conn.SCOT_DADOS
.Where(r => !conn.SCOT_DADOS.Any(r2 => r2.USER == r.USER && r2.Date > r.Date))
// end of Db Query
.AsEnumerable()
.Select(r => new
{
Name = svc.GetUserName(r.User),
Value = r.Value,
Date = r.Date.ToString("dd/MM/yyyy HH:mm:ss")
}).ToList();

I'm a bit confused but from your attempts with First() think you mean this:
conn.SCOT_DADOS.GroupBy(item => item.User)
.Select(grp => grp.OrderByDescending(i => t.Date).First());
This will retrieve for each User only the latest record of it
The reason only swapping the GroupBy and OrderByDescending isn't enough and that you need the Select is that once you grouped that data your enumerable is IEnumerable<IGrouping<User,YourType>>. Each IGrouping is actually a collection by itself so you need to Select only the 1 item you want from it.
Another way is to replace the Select with:
.SelectMany(grp => grp.OrderByDescending(i => t.Date).Take(1))
IMO the first is cleaner, but the second is in the case you need for each user N first items
On the query above you can also add what you have in the foreach loop:
conn.SCOT_DADOS.GroupBy(item => item.User)
.Select(grp => grp.OrderByDescending(i => t.Date).First())
.AsEnumerable()
.Select(item => new {
Name = svc.GetUserName(item.User),
Value = item.Value,
Date = item.Date.ToString("dd/MM/yyyy HH:mm:ss")
}).ToList();
The use of the AsEnumerable() is to invoke the query to be executed to the database before the last Select() which uses the GetUserName method that will not be known to the Oracle database
IMO representing the DateTime as string is not a good way..
Update - The error you get:
Oracle 11.2.0.3.0 does not support apply
It seems that as for this version of Oracle it does not support GroupBy with Select via linq. See Linq to Entities Group By (OUTER APPLY) “oracle 11.2.0.3.0 does not support apply”.
One answer there recommended to create a view in the database for this and then use linq to select over that view. That is what I'd go for

Try this
conn.SCOT_DADOS.GroupBy(x => x.User).Select(x => new
{
User = x.Key,
Date = list.Where(y => y.User == x.Key).Max(y => y.Date)
});

Linq Group By not taking inner entity

I am using entity framework and doing a group by over a table. My query is a follows:-
var brokerPaymentLists = dbContext.BrokerPayments
.Include("PaymentDetail")
.Where(bp => bp.IdPaymentStatus == (long)EntityModel.Additions.Variables.PaymentStatus.ALLOTED)
.GroupBy(bp => bp.IdBroker,
(key, g) => new
{
IdBroker = key.Value,
BrokerPayments = g.ToList()
}).ToList();
I have included PaymentDetail but after grouping by i can see that the paymentdetail for each item in the BrokerPayments i null. Any suggestion why this is the case, also how can i do the group by such that I can my my paymentDetail insisde each of the BrokerPayments;

The eagerly loading by using Include requires the shape of the data to do not be changed since the Include is applied. In your case this means the query must return IQueryable<BrokerPayments>. But the GroupBy operator changes the shape because it returns IQueryable<IGrouping<TKey, TSource>>. Same will happen with projections and custom joins.
As a workaround you can execute grouping in LINQ to Objects like:
var brokerPaymentLists = dbContext.BrokerPayments
.Include("PaymentDetail")
.Where(bp => bp.IdPaymentStatus == (long)EntityModel.Additions.Variables.PaymentStatus.ALLOTED)
.AsEnumerable()
.GroupBy(bp => bp.IdBroker,
(key, g) => new
{
IdBroker = key.Value,
BrokerPayments = g
});
NOTE: pay attention that the query exectuion will not be defferd

How to filter entity framework result with multiple columns using a lambda expression

I have the following table:
And the following data:
How can i filter the result, so that i only get the latest row from each omraade_id (sorted descending by timestamp)?
Which in this case would be the rows with id: 1010 and 1005
--
From #lazyberezovsky's answer, i have created the following expression:
dbConnection = new ElecEntities();
var query = from data in dbConnection.Valgdata
orderby data.timestamp descending
group data by data.omraade_id into g
select g.FirstOrDefault();
return query.ToList();
It returns two rows with the ID 3 and 4, which are the first two rows in the database, and also the ones with the lowest timestamp. Any idea why?

var query = dbConnection.Valgdata
.GroupBy(x => x.omraade_id)
.Select(g => g
.OrderByDescending(x => x.timestamp)
.FirstOrDefault());

I have no experience with EF, so I'm unsure if only SQL-esque linq works here. A plain C#-ish:
var query = dbConnection.Valgdata.GroupBy(u => u.omraade_id)
.Select(x => x.FirstOrDefault(y => x.Max(p => p.timestamp) == y.timestamp));

You have put filter on every item. It should be applied on complete query result, not on every item.
Following is updated query.
var query = (from data in dbConnection.Valgdata
orderby data.timestamp descending
group data by data.omraade_id into g
select g).FirstOrDefault();

var query = from v in dbConnection.Valgdata
orderby v.timestamp descending
group v by v.omraade_id into g
select g.First();
This will return only record with max timestamp for each omraade_id.
UPDATE query above works fine to me (at least for MS SQL Linq provider). Also you don't need to do FirstOrDefault - if omraade_id is grouped, then it definitely has at least one row.
var query = from v in dbConnection.Valgdata
group v by v.omraade_id into g
select g.OrderByDesc(x => x.timestamp).First();

This is my solution so far:
var data = dbConnection.Valgdata.Where(x => x.godkendt == false).ToList();
var dataGrouped = data.GroupBy(x => x.omraade_id).ToList();
List<Valgdata> list = new List<Valgdata>();
foreach (var grpdata in dataGrouped)
{
var dataGroup = grpdata.OrderByDescending(x => x.timestamp).ToList();
list.Add(dataGroup.FirstOrDefault());
}
return list;
I dont know if it is the most effective, but it works.