We Keep Coding

Select latest records and add results as new columns

I have the following database
my StudentRecords for specific sutdent is
How can i get the latest 3 studentRecords based on the latest Records.RecordDate
so for each sutdent, row will have ReportItems.ReportItemName ReportItems.TotalPSR,, Students.FullName, + add the following columns:
The latest studentRecords based on RecordDate and display the psr + reordsNames + recordDate so that one row look like this
Is this even possible?
And how can i do this using EF6 also what will be the SQL select statment

Providing your EF entities are set up with their mappings correctly to associate the Students, StudentRecords, and Records then you should be able to accomplish this in EF using something like the following:
var results = testContext.Students
.SelectMany(s => s.StudentRecords.OrderByDescending(sr => sr.Record.RecordDate).Take(3))
.GroupBy(x => x.Student)
.ToList();
What this will give you is a result per-student in a grouped structure where the grouping is on the student, with the grouped values are StudentResults. From there you can extract the student details from the "Key", while each result is a collection of (up to) 3 latest StudentRecords in descending order. You will need to handle the scenario where the # of results is < 3.
As a simple example extraction...
foreach (var studentGrouping in results)
{
studentName = studentGrouping.Key.FullName;
psr1 = studentGrouping[0].PSR;
recordName1 = studentGrouping[0].Record.RecordName;
recordDate1 = studentGrouping[0].Record.RecordDate;
// ... etc.
}
Now a caveat to the above solution is that it will invoke lazy-loading on the Records, so up to 3 hits per student. Curiously I tried to mitigate this with .Include(s=>s.StudentRecords.Select(r=>r.Record)) or trying the .Include(r=> r.Record) after the SelectMany() but while the initial query looked to include the columns for the Records, it still triggered the lazy loads.
If anyone can offer a correction to address the lazy loads, I'd love to hear it. I was a bit surprised to see them after adding the Include statements.
If your object model is more complex, such as lots of fields in Student/Record, etc. then I would recommend altering the above to return an anonymous type structure to retrieve just the values you're going to need to populate your results. The above is Ok for simple structures.
For example if Student had 30 columns, but we only care about the ID and Name:
var results = testContext.Students
.SelectMany(s => s.StudentRecords.OrderByDescending(sr => sr.Record.RecordDate).Take(3))
.Select( sr=> new {sr.Student.StudentId, sr.Student.FullName, sr.PSR, sr.Record.RecordId, RecordName = sr.Record.Name})
.GroupBy(x => new{StudentId, FullName})
.ToList();
This would return a structure where the Key was an anonymous type containing just the StudentId and Name, with the grouped values being the type containing the record details for that student. This has the benefit of only returning the data you will need in a single query to the database.

You can use Row_Number and Pivot as below:
;With Cte as (
Select RI.ReportItemName, RI.TotalPSR, S.FullName, SR.PSR, R.[Name], R.[Date],
RowNPSR = Row_Number() over(Partition by SR.StudentRecord order by R.RecordDate DESC),
RowName = Row_Number() over(Partition by SR.StudentRecord order by R.RecordDate DESC) + 500,
RowDate = Row_Number() over(Partition by SR.StudentRecord order by R.RecordDate DESC) + 1000
from
StudentRecords SR
Join Students s
on SR.StudentId = S.StudentId
Join Records R
On SR.RecordId = R.recordId
Left Join ReportItems RI
On S.ReportItemId = RI.ReportItemId
)
Select a.*, p1.[1] as PSR1, p1.[2] as PSR2, p1.[3] as PSR3
, p2.[501] as Name1, p2.[502] as Name2, p2.[503] as Name3 from (
Select * from CTE RowNPSR <= 3 ) a
Pivot (max(PSR) for RowNPSR in ([1],[2],[3]) ) p1
Pivot (max([Name]) for RowName in ([501],[502],[503]) ) p2
Pivot (max([Date]) for RowDate in ([1001],[1002],[1003]) ) p3

entity framework linq query, get top N postIDs from favorite's table based on count

I have a database for keeping track of Favorites that looks like
[ID]
[PostID]
[userID]
[DateFavorited]
I'm attempting to write an entity framework query in Linq that will get the 12 PostIDs that appear the most often in this database.
I've looked at the documentation, but I'm just not putting together how to do this. The purpose is for a "most Favorited" page
I feel like there is an elegant solution, but I've frustrated myself to the point that I can't think of the way to do it without feting the entire table that that's a bad idea.

In SQL, this would be:
SELECT
TOP 12
PostID,
COUNT(*) As FavCount
FROM
Favorites
GROUP BY
PostID
ORDER BY
FavCount DESC
In Linq, I believe it would be:
var ret = db.Favorites.GroupBy(
fav => fav.PostId
).Select(
favGroup => new {
PostId = favGroup.Key,
Count = favGroup.Count()
}
).OrderBy(
row => row.Count
).Take( 12 );
With the type of ret being IEnumerable<Anonymous{ PostId, Count }>.

EF6 Getting Count In Single Call

Try to do a single database call to get an entity, as well as the count of related child entities.
I know I can retrieve the count using
var count = Context.MyChildEntitySet.Where(....).Count();
or even MyEntity.ListNavigationProperty.Count()
But That means getting the entity first, followed by another call in order to get the count or use an Include which would retrieve the whole list of related entities.
I am wondering is it possible to add a "Computed" column in SQL Server to return the Count of related rows in another table?
If not how do I ask EF to retrieve the related count for each entity in once call?
I am thinking of possibly using Join with GroupBy, but this seems an Ugly solution/hack.
public class MyEntity
{
public uint NumberOfVotes{ get; private set; }
}
which ideally woudl generate SQL Similar to:
SELECT
*,
(SELECT Count(*) FROM ChildTable c WHERE c.ParentId = p.Id) NumberOfVotes
FROM ParentTable p

UPDATED
You can always drop down to using actual SQL in the following way...
string query = "SELECT *,
(SELECT Count(*) FROM ChildTable c
WHERE c.ParentId = p.Id) as NumberOfVotes
FROM ParentTable p";
RowShape[] rows = ctx.Database.SqlQuery<RowShape>(query, new object[] { }).ToArray();
I realize this is not ideal because then you are taking a dependency on the SQL dialect of your target database. If you moved form SQL Server to something else then you would need to check and maybe modify your string.
The RowShape class is defined with the properties that match the ParentTable along with an extra property called NumberOfVotes that has the calculated count.
Still, a possible workaround.

SQL Row_Number() and Linq record count different

I have two bits of code doing basically the same thing but the results are different. I am trying to get the index number of a record from an ordered list in the database, and then skip/take some results.
Linq method:
int index = db.users.OrderByDescending(o => o.TotalScore)
.ToList()
.FindIndex(f => f.UserId == User.UserId);
// returns 557
SQL method:
WITH IndexedUsers AS (
SELECT UserId,
ROW_NUMBER() OVER (ORDER BY TotalScore desc) AS 'RowNumber'
FROM [User]
)
SELECT RowNumber
FROM IndexedUsers
WHERE UserId = #userId
//Returns 559
Stumped at the difference here. I dont want to be calling ToList on the user collection from the database, i am just using it for testing purposes. I would expect the results to be 1 out with the FindIndex method being 0 based. Any ideas why the values aren't the same?

Linq2Sql: Get every N'th row [duplicate]

Anybody know how to write a LINQ to SQL statement to return every nth row from a table? I'm needing to get the title of the item at the top of each page in a paged data grid back for fast user scanning. So if i wanted the first record, then every 3rd one after that, from the following names:
Amy, Eric, Jason, Joe, John, Josh, Maribel, Paul, Steve, Tom
I'd get Amy, Joe, Maribel, and Tom.
I suspect this can be done... LINQ to SQL statements already invoke the ROW_NUMBER() SQL function in conjunction with sorting and paging. I just don't know how to get back every nth item. The SQL Statement would be something like WHERE ROW_NUMBER MOD 3 = 0, but I don't know the LINQ statement to use to get the right SQL.

Sometimes, TSQL is the way to go. I would use ExecuteQuery<T> here:
var data = db.ExecuteQuery<SomeObjectType>(#"
SELECT * FROM
(SELECT *, ROW_NUMBER() OVER (ORDER BY id) AS [__row]
FROM [YourTable]) x WHERE (x.__row % 25) = 1");
You could also swap out the n:
var data = db.ExecuteQuery<SomeObjectType>(#"
DECLARE #n int = 2
SELECT * FROM
(SELECT *, ROW_NUMBER() OVER (ORDER BY id) AS [__row]
FROM [YourTable]) x WHERE (x.__row % #n) = 1", n);

Once upon a time, there was no such thing as Row_Number, and yet such queries were possible. Behold!
var query =
from c in db.Customers
let i = (
from c2 in db.Customers
where c2.ID < c.ID
select c2).Count()
where i%3 == 0
select c;
This generates the following Sql
SELECT [t2].[ID], [t2]. --(more fields)
FROM (
SELECT [t0].[ID], [t0]. --(more fields)
(
SELECT COUNT(*)
FROM [dbo].[Customer] AS [t1]
WHERE [t1].[ID] < [t0].[ID]
) AS [value]
FROM [dbo].[Customer] AS [t0]
) AS [t2]
WHERE ([t2].[value] % #p0) = #p1

Here's an option that works, but it might be worth checking that it doesn't have any performance issues in practice:
var nth = 3;
var ids = Table
.Select(x => x.Id)
.ToArray()
.Where((x, n) => n % nth == 0)
.ToArray();
var nthRecords = Table
.Where(x => ids.Contains(x.Id));

Just googling around a bit I haven't found (or experienced) an option for Linq to SQL to directly support this.
The only option I can offer is that you write a stored procedure with the appropriate SQL query written out and then calling the sproc via Linq to SQL. Not the best solution, especially if you have any kind of complex filtering going on.

There really doesn't seem to be an easy way to do this:
How do I add ROW_NUMBER to a LINQ query or Entity?
How to find the ROW_NUMBER() of a row with Linq to SQL
But there's always:
peopleToFilter.AsEnumerable().Where((x,i) => i % AmountToSkipBy == 0)
NOTE: This still doesn't execute on the database side of things!

This will do the trick, but it isn't the most efficient query in the world:
var count = query.Count();
var pageSize = 10;
var pageTops = query.Take(1);
for(int i = pageSize; i < count; i += pageSize)
{
pageTops = pageTops.Concat(query.Skip(i - (i % pageSize)).Take(1));
}
return pageTops;
It dynamically constructs a query to pull the (nth, 2*nth, 3*nth, etc) value from the given query. If you use this technique, you'll probably want to create a limit of maybe ten or twenty names, similar to how Google results page (1-10, and Next), in order to avoid getting an expression so large the database refuses to attempt to parse it.
If you need better performance, you'll probably have to use a stored procedure or a view to represent your query, and include the row number as part of the stored proc results or the view's fields.