How to select the third top record in LINQ - linq

Instead of select the top 3 records, I hope to select the top 3rd record in a single query.

Assuming your query already defines the right order in your result set, you can just skip the first two using Skip() then finally take the third result, or null if it doesn't exist (less than 3 results):
var result = query.Skip(2).Take(1).SingleOrDefault();
Edit:
Actually this can be done shorter, since the Take(1) and SingleOrDefault() can be expressed with FirstOrDefault() instead:
var result = query.Skip(2).FirstOrDefault();

Related

Strange behaviour when using FILTER to filter a different table with no direct relationship?

I have two facts tables, First and Second, and two dimension tables, dimTime and dimColour.
Fact table First looks like this:
and facet table Second looks like this:
Both dim-tables have 1:* relationships to both fact tables and the filtering is one-directional (from dim to fact), like this:
dimColour[Color] 1 -> * First[Colour]
dimColour[Color] 1 -> * Second[Colour]
dimTime[Time] 1 -> * First[Time]
dimTime[Time] 1 -> * Second[Time_]
Adding the following measure, I would expect the FILTER-functuion not to have any affect on the calculation, since Second does not filter First, right?
Test_Alone =
CALCULATE (
SUM ( First[Amount] );
First[Alone] = "Y";
FILTER(
'Second';
'Second'[Colour]="Red"
)
)
So this should evaluate to 7, since only two rows in First have [Alone] = "Y" with values 1 and 6 and that there is no direct relationship between First and Second. However, this evaluates to 6. If I remove the FILTER-function argument in the calculate, it evaluates to 7.
There are thre additional measures in the pbix-file attached which show the same type of behaviour.
How is filtering one fact table which has no direct relationship to a second fact table affecting the calculation done on the second table?
Ziped Power BI-file: PowerBIFileDownload
Evaluating the table reference 'Second' produces a table that includes the columns in both the Second table, as well as those in all the (transitive) parents of the Second table.
In this case, this is a table with all of the columns in dimColour, dimTime, Second.
You can't see this if you just run:
evaluate 'Second'
as when 'evaluate' returns the results to the user, these "Parent Table" (or "Related") columns are not included.
Even so, these columns are certainly present.
When a table is converted to a row context, these related columns become available via RELATED.
See the following queries:
evaluate FILTER('Second', ISBLANK(RELATED(dimColour[Color])))
evaluate 'Second' order by RELATED(dimTime[Hour])
Similarly, when arguments to CALCULATE are used to update the filter context, these hidden "Related" columns are not ignored; hence, they can end up filtering First, in your example. You can see this, by using a function that strips the related columns, such as INTERSECT:
Test_ActuallyAlone = CALCULATE (
SUM ( First[Amount] ),
First[Alone] = "Y",
//This filter now does nothing, as none of the columns in Second
//have an impact on 'SUM ( First[Amount] )'; and the related columns
//are removed by the INTERSECT.
FILTER(
INTERSECT('Second', 'Second')
'Second'[Colour]="Red"
)
)
(See these resources that describe the "Expanded Table"
(this is an alternative but equivalent explanation of this behaviour)
https://www.sqlbi.com/articles/expanded-tables-in-dax/
https://www.sqlbi.com/articles/context-transition-and-expanded-tables/
)

Get n-th result returned from MATCH query in Neo4J database

I've set up a library database where users borrow books. Using a MATCH Command i can return the book titles and number of their lendings by descending order.
My Cypher for returning the list of books and number of lendings is:
MATCH (user)-[:LENDING]->(b:Book)
RETURN b.title, COUNT(b.title) as numberOfRents
ORDER BY numberOfRents DESC
This is working properly. However, i need to get the n-th book(by lendings) returned only(let's say the third for example), which is something i failed to do until now.
Sounds like you need SKIP and LIMIT
MATCH (user)-[:LENDING]->(b:Book)
RETURN b.title, COUNT(b.title) as numberOfRents
ORDER BY numberOfRents DESC
SKIP 2 LIMIT 1
// skips the first 2, so you only get the 3rd

Laravel sum on ->first() sums more than 1 result

I have the following eloquent query
$raw = Model::select('out', 'in')->orderBy('created_at', 'DESC')->first();
That returns a collection of a single item, where Out = 0.0 and In = 90.0.
If I then do this:
$sumO = $raw->sum('out');
$sumI = $raw->sum('in');
I get $sumO = 13,651.41 and $sumI = 13371.69
I don't understand, because those sums don't even equal the sum of my entire table for those colums.
But it seems like->sum() is being called on the entire table/query instead of just the first result like I thought it would.
Now, I know sum of a single row is weird, and I'm not actually doing this in production. I just want to know what it is doing.
Shouldn't it still just sum the 1 number to equal itself?
It's just one row when using ->first(), so there's no need to use ->sum() just use $raw->in and $raw->out.
Also, ->sum() used with a single column at a time.

Linq Query Where Contains

I'm attempting to make a linq where contains query quicker.
The data set contains 256,999 clients. The Ids is just a simple list of GUID'S and this would could only contain 3 records.
The below query can take up to a min to return the 3 records. This is because the logic will go through the 256,999 record to see if any of the 256,999 records are within the List of 3 records.
returnItems = context.ExecuteQuery<DataClass.SelectClientsGridView>(sql).Where(x => ids.Contains(x.ClientId)).ToList();
I would like to and get the query to check if the three records are within the pot of 256,999. So in a way this should be much quicker.
I don't want to do a loop as the 3 records could be far more (thousands). The more loops the more hits to the db.
I don't want to grap all the db records (256,999) and then do the query as it would take nearly the same amount of time.
If I grap just the Ids for all the 256,999 from the DB it would take a second. This is where the Ids come from. (A filtered, small and simple list)
Any Ideas?
Thanks
You've said "I don't want to grab all the db records (256,999) and then do the query as it would take nearly the same amount of time," but also "If I grab just the Ids for all the 256,999 from the DB it would take a second." So does this really take "just as long"?
returnItems = context.ExecuteQuery<DataClass.SelectClientsGridView>(sql).Select(x => x.ClientId).ToList().Where(x => ids.Contains(x)).ToList();
Unfortunately, even if this is fast, it's not an answer, as you'll still need effectively the original query to actually extract the full records for the Ids matched :-(
So, adding an index is likely your best option.
The reason the Id query is quicker is due to one field being returned and its only a single table query.
The main query contains sub queries (below). So I get the Ids from a quick and easy query, then use the Ids to get the more details information.
SELECT Clients.Id as ClientId, Clients.ClientRef as ClientRef, Clients.Title + ' ' + Clients.Forename + ' ' + Clients.Surname as FullName,
[Address1] ,[Address2],[Address3],[Town],[County],[Postcode],
Clients.Consent AS Consent,
CONVERT(nvarchar(10), Clients.Dob, 103) as FormatedDOB,
CASE WHEN Clients.IsMale = 1 THEN 'Male' WHEN Clients.IsMale = 0 THEN 'Female' END As Gender,
Convert(nvarchar(10), Max(Assessments.TestDate),103) as LastVisit, ";
CASE WHEN Max(Convert(integer,Assessments.Submitted)) = 1 Then 'true' ELSE 'false' END AS Submitted,
CASE WHEN Max(Convert(integer,Assessments.GPSubmit)) = 1 Then 'true' ELSE 'false' END AS GPSubmit,
CASE WHEN Max(Convert(integer,Assessments.QualForPay)) = 1 Then 'true' ELSE 'false' END AS QualForPay,
Clients.UserIds AS LinkedUsers
FROM Clients
Left JOIN Assessments ON Clients.Id = Assessments.ClientId
Left JOIN Layouts ON Layouts.Id = Assessments.LayoutId
GROUP BY Clients.Id, Clients.ClientRef, Clients.Title, Clients.Forename, Clients.Surname, [Address1] ,[Address2],[Address3],[Town],[County],[Postcode],Clients.Consent, Clients.Dob, Clients.IsMale,Clients.UserIds";//,Layouts.LayoutName, Layouts.SubmissionProcess
ORDER BY ClientRef
I was hoping there was an easier way to do the Contain element. As the pool of Ids would be smaller than the main pool.
A way I've speeded it up for now is. I've done a Stinrg.Join to the list of Ids and added them as a WHERE within the main SQL. This has reduced the time down to a seconds or so now.

Linq Paging - How to incorporate total record count

I am trying to figure out the best way of getting the record count will incorporating paging. I need this value to figure out the total page count given a page size and a few other variables.
This is what i have so far which takes in the starting row and the page size using the skip and take statements.
promotionInfo = (from p in matches
orderby p.PROMOTION_NM descending
select p).Skip(startRow).Take(pageSize).ToList();
I know i could run another query, but figured there may be another way of achieving this count without having to run the query twice.
Thanks in advance,
Billy
I know i could run another query, but figured there may be another way of achieving this count without having to run the query twice.
No, you have to run the query.
You can do something like:
var source = from p in matches
orderby p.PROMOTION_NM descending
select p;
var count = source.Count();
var promotionInfo = source.Skip(startRow).Take(pageSize).ToList();
Be advised, however, that Skip(0) isn't free.

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