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I didn't true use order by in linq

Hi i develop web app with c#. I have sql query and i convert to linq but it's not working true because of order by
My sql query
Select TOP 3 HastalikIsmi From Hastaliklar group by HastalikIsmi order by Count(*) desc
My linq
public List<HastalikDto> GetHastalikDto()
{
using (SirketDBContext context = new SirketDBContext())
{
var result = from hastalik in context.Hastaliklar
group hastalik by hastalik.HastalikIsmi into isim
select new HastalikDto { HastalikIsmi = isim.Key };
return result.OrderBy(h => h.HastalikIsmi).Take(3).ToList();
}
}

Here's how you can do the order by on the count of each group and take the 3 with the highest count.
var result = context.Hastaliklar
.GroupBy(x => x.HastalikIsmi)
.OrderByDescending(grp => grp.Count())
.Select(grp => grp.Key)
.Take(3)
.ToList();

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.

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()
} );

How to create query of queries using LINQ?

I'm trying to convert query of queries used in ColdFusion to LINQ and C#. The data come from data files, rather than from the database.
I converted the first query, but have no clue as to
how to use it to query the second query.
how to include count(PDate) as DayCount in the second query.
Below is the code using query of queries in ColdFusion:
First query
<cfquery name="qSorted" dbtype = "query">
SELECT OA, CD,PDate,
FROM dataQuery
GROUP BY CD,OA,PDate,
</cfquery>
Second query
<cfquery name="qDayCount" dbtype = "query">
SELECT OA, CD, count(PDate) as DayCount
FROM qSorted // qSorted is from the first query.
GROUP BY
OA, CD
ORDER BY
OA, CD
</cfquery>
Here's the first converted LINQ query, and it works fine:
var Rows = allData.SelectMany(u => u._rows.Select(t => new
{
OA = t[4],
CD = t[5],
PDate = t[0]
}))
.GroupBy(x => new { x.CD, x.OA, x.PDate })
.Select(g => new
{
g.Key.OA,
g.Key.CD,
g.Key.PDate
})
.ToList();
Here's the pseudo-code for the second LINQ query, which I need your assistance:
var RowsDayCount = Rows //Is this correct? If not, how to do it?
.GroupBy(x => new { x.OA, x.PDate, x.CD, })
.Select(g => new
{
g.Key.OA,
g.Key.CD,
g.Key.PDate,//PDate should be PDate.Distinct().Count() asDayCount
// See DayCount in cfquery name="qDayCount" above.
})
.OrderBy(u => u.OA)
.ThenBy(u => u.CD)
.ToList();

Your second query origionally wasn't grouping on PDate, but your translation is. That's wrong. If you want to count the number of PDates for each OA/CD pair, you need to not group on PDate. Once you've made that change, you can modify the Select to pull out all of the PDate values from the group, and count the distinct values.
.GroupBy(x => new { x.OA, x.CD, })
.Select(g => new
{
g.Key.OA,
g.Key.CD,
DayCount = g.Select(item => item.PDate).Distinct().Count(),
})

Weird LINQ to SQL Timeout Issue using FirstOrDefault()

I have the following code which times out:
using (var ts = new TransactionScope(TransactionScopeOption.Required, new TransactionOptions { IsolationLevel = System.Transactions.IsolationLevel.ReadUncommitted }))
{
ECWSDataContext dc = new ECWSDataContext();
IQueryable<Ticket> results = dc.Tickets;
Business.TicketStatistic statistic = results
.Select(r => new
{
GroupID = 1,
IsVoided = r.IsVoided ? 1 : 0,
IsWarning = r.TicketFilingTypeID == 5 ? 1 : 0,
TotalFelonies = r.TotalFelonies,
TotalMisdemeanors = r.TotalMisdemeanors,
TotalInfractions = r.TotalInfractions,
TotalOrdinances = r.TotalOrdinances,
TotalWarnings = r.TotalWarnings
})
.GroupBy(t => t.GroupID)
.Select(g => new Business.TicketStatistic()
{
TotalTickets = g.Count(),
TotalVoids = g.Sum(x => x.IsVoided),
TotalTicketWarnings = g.Sum(x => x.IsWarning),
TotalFelonies = g.Sum(x => x.TotalFelonies),
TotalMisdemeanors = g.Sum(x => x.TotalMisdemeanors),
TotalInfractions = g.Sum(x => x.TotalInfractions),
TotalOrdinances = g.Sum(x => x.TotalOrdinances),
TotalOffenseWarnings = g.Sum(x => x.TotalWarnings)
}).FirstOrDefault();
}
I profiled the SQL using SQL Server Profiler and grabbed the executed SQL. As expected, it contains a TOP 1. When I run the exact SQL in SQL Management Studio, it comes back in no time at all. Yet, it continues to timeout in the code. Amazingly, changing it to the following works just fine:
using (var ts = new TransactionScope(TransactionScopeOption.Required, new TransactionOptions { IsolationLevel = System.Transactions.IsolationLevel.ReadUncommitted }))
{
ECWSDataContext dc = new ECWSDataContext();
IQueryable<Ticket> results = dc.Tickets;
var stats = results
.Select(r => new
{
GroupID = 1,
IsVoided = r.IsVoided ? 1 : 0,
IsWarning = r.TicketFilingTypeID == 5 ? 1 : 0,
TotalFelonies = r.TotalFelonies,
TotalMisdemeanors = r.TotalMisdemeanors,
TotalInfractions = r.TotalInfractions,
TotalOrdinances = r.TotalOrdinances,
TotalWarnings = r.TotalWarnings
})
.GroupBy(t => t.GroupID)
.Select(g => new Business.TicketStatistic()
{
TotalTickets = g.Count(),
TotalVoids = g.Sum(x => x.IsVoided),
TotalTicketWarnings = g.Sum(x => x.IsWarning),
TotalFelonies = g.Sum(x => x.TotalFelonies),
TotalMisdemeanors = g.Sum(x => x.TotalMisdemeanors),
TotalInfractions = g.Sum(x => x.TotalInfractions),
TotalOrdinances = g.Sum(x => x.TotalOrdinances),
TotalOffenseWarnings = g.Sum(x => x.TotalWarnings)
}).ToArray();
Business.TicketStatistic statistic = stats.FirstOrDefault();
}
I understand that now I am enumerating the results before applying the FirstOrDefault() to the now in-memory collection. But it seems strange that executing the same SQL output in the first scenario directly in SQL Server had no problems.
Can somebody maybe explain what is going on here? In this instance, it was a group query that always returned one row regardless. So I am lucky that I can enumerate before applying FirstOrDefault(). But for possible future reference, what if that query returned thousands of rows to which I only wanted the TOP 1.
ADDITION INFO
The SQL using .FirstOrDefault():
SELECT TOP 1 Field1, Field2...
FROM
(
SELECT SUM(Field) as Field1, ...
FROM ...
) SUB
The SQL using .ToArray():
SELECT SUM(Field) as Field1, ...
FROM ...
Executing either directly in SQL Mgt Studio resulted in the same results in the same amount of time. However, when LINQ executes the first one, I get a timeout.

This is a common problem when using linq to sql. If you think about sql, when you do a group by and then a firstordefault you're asking sql to aggregate and then unaggregate. It's hard for sql to deal with the individual elements in a group by since it'll be doing multiple queries to reach the individual elements.
When you do ToArray, you're actually pulling the data back into memory and the group by is actually stored in memory with the individual elements so reaching these will be a lot faster.