I'm trying to select the top five most frequent values and their count in my table and return them in a Dictionary. I'm able to get the values in sql:
SELECT top 5
SR_Status,
COUNT(SR_Status) AS 'value_count'
FROM
ServiceRequests
GROUP BY
SR_Status
ORDER BY
'value_count' DESC;
How to convert to linq and assign to Dictionary
You do not specify if you are using Linq2Sql or Linq2Objects, so, let's just assume linq then. Try something like this (see the comments for each line):
var result = (from s in ServiceRequests // define the data source
group s by s.SR_Status into g // group all items by status
orderby g.Count() descending // order by count descending
select new { g.Key, Total = g.Count() }) // cast the output
.Take(5) // take just 5 items
.ToDictionary(x => x.Key, x => x.Total); // cast to dictionary
Obs: I didn't test it.
Assuming you are using Entity Framework and have an EntitySet named ServiceRequests and all the properties names are the same as the column names:
var result = context.ServiceRequests.GroupBy(sr => sr.SR_Status)
.Select(g => new { Key = g.Key, Count = g.Count() })
.OrderByDescending(kv => kv.Count)
.Take(5)
.ToList();
Related
I am using Entity Framework in .NET 7.
I have 3 entities:
Course that contains a ProfessorId among other things
Grade that has a CourseId among other things
Professor
I want to get all the courses that are assigned to a professor and have at least 1 grade associated with them and filter them in a Dictionary<string, CourseViewModel> where string is the semester.
I have written the following LINQ query:
var professorGradedCourses = _dbContext.Courses
.Where(course => course.ProfessorId == professorId && course.Grades.Any())
.Select(course => new CourseViewModel
{
Title = course.Title,
Semester = course.Semester,
})
.GroupBy(course => course.Semester)
.OrderBy(course => course.Key)
.ToDictionary(group => group.Key, group => group.ToList());
When that executes I get an exception saying it can't be translated.
If I remove the OrderBy and keep only the GroupBy, it works and the translated SQL in Microsoft SQL Server is:
SELECT [c].[Semester], [c].[Title]
FROM [Courses] AS [c]
WHERE [c].[ProfessorId] = #__professorId_0
AND EXISTS (SELECT 1
FROM [Grades] AS [g]
WHERE [c].[Id] = [g].[CourseId])
ORDER BY [c].[Semester]
As you can see it adds ORDER BY anyway, even though I have removed it and kept only GroupBy(). Can someone explain why is that? What if I wanted to order by descending would that be possible? Also the weird thing is that if I remove GroupBy() and keep only OrderBy() and replace the ToDictionary with ToList, it works and the exact same query is produced (only now I can't really use the results without further actions).
LINQ GroupBy :
Groups the elements of a sequence.
SQL GROUP BY :
A SELECT statement clause that divides the query result into groups of rows, usually by performing one or more aggregations on each group. The SELECT statement returns one row per group.
They aren't equivalent. The main difference is LINQ GroupBy return a collection by key, when SQL GROUP BY return ONE element (column) by key.
If the projection ask ONE element by key, then EF Core translate LINQ GroupBy to SQL GROUP BY :
// Get the number of course by semester
context
.Courses
.GroupBy(c => c.Semester)
.Select(cs => new { Semester = cs.Key, Count = cs.Count() })
.ToList();
Translated to :
SELECT [c].[Semester], COUNT(*) AS [Count]
FROM [Courses] AS [c]
GROUP BY [c].[Semester]
But if the projection ask several element, then EF Core translate LINQ GroupBy to SQL ORDER BY and group by itself.
context
.Courses
.Select(c => new { c.Id, c.Semester })
.GroupBy(c => c.Semester)
.ToDictionary(cs => cs.Key, cs => cs.ToList());
Translated to :
SELECT [c].[Semester], [c].[Id]
FROM [Courses] AS [c]
ORDER BY [c].[Semester]
If the result is :
Semester
Id
2023 S1
1
2023 S1
4
2023 S2
2
...
...
Then EF Core read like :
Read first row : Semester is "2023 S1"
No group
Then create a group and add the row in.
Read second row : Semester is "2023 S1"
The key is the same that precedent element
Then Add the row in the group
Read the third row : Semester is "2023 S2"
The key is different that precedent element
Then create a new group and the row in.
And so on...
You understand the interest of sorting.
About the error, I don't know that EF Core can't. The query sound legit. Maybe this should not be implemented at this time.
About that you try, to convert a sorted grouping enumeration to a dictionary. This is weird because the dictionary isn't sortable. Then this sorts elements and put them in loose.
If Dictionary seem sorted, it's a coincidence, not a feature. In intern, the dictionary sort element by key's has code, that is generally the sorted order... But not every time.
If you want a sorted dictionary, you can use SortedDictyonary. But it can be tricky if you need a custom sort rule, like :
context
.Courses
.Select(c => new { c.Id, c.Semester })
.GroupBy(c => c.Semester)
.ToImmutableSortedDictionary(cs => cs.Key, cs => cs.ToList(), new ReverseComparer<string>());
public class ReverseComparer<T> : IComparer<T>
{
private IComparer<T> _comparer = Comparer<T>.Default;
public int Compare(T? x, T? y)
{
return _comparer.Compare(x, y) * -1;
}
}
The exception you are encountering is most likely due to the fact that the OrderBy clause cannot be translated into SQL by Entity Framework. The OrderBy clause is executed in memory after the data has been retrieved from the database, which is why it works when you remove it and keep only the GroupBy clause.
However, if you want to order the dictionary by descending, you can simply call the Reverse method on the ToDictionary result:
var professorGradedCourses = _dbContext.Courses
.Where(course => course.ProfessorId == professorId && course.Grades.Any())
.Select(course => new CourseViewModel
{
Title = course.Title,
Semester = course.Semester,
})
.GroupBy(course => course.Semester)
.OrderByDescending(course => course.Key)
.ToDictionary(group => group.Key,
group => group.ToList())
.Reverse();
This way, the dictionary will be sorted in descending order based on the semester.
Give this a try and let me know how it works for you.
EDIT:
Converting the IEnumerable back to a Dictionary should work like this:
var professorGradedCourses = _dbContext.Courses
.Where(course => course.ProfessorId == professorId && course.Grades.Any())
.Select(course => new CourseViewModel
{
Title = course.Title,
Semester = course.Semester,
})
.GroupBy(course => course.Semester)
.OrderByDescending(course => course.Key)
.ToDictionary(group => group.Key,
group => group.ToList())
.Reverse()
.ToDictionary(pair => pair.Key,
pair => pair.Value);
When I run this expression i can see that the list order is correctly in sequence by the highest ActionId's
var list = db.Actions.Where(z => z.RunId
== RunId).OrderByDescending(w =>
w.ActionId).ToList();
I only want to select the highest ActionId's of each ActionName so I now do:
var list = db.Actions.Where(z => z.RunId
== RunId).OrderByDescending(w =>
w.ActionId).GroupBy(c => new
{
c.ActionName,
c.MachineNumber,
})
.Select(y =>
y.FirstOrDefault()).ToList();
When I look at the contents of list, it hasn't selected the ActionName/MachineNumber with the highest ActionId, which I assumed would be the case by ordering then using FirstOrDefault().
Any idea where I'm going wrong? I want to group the records by the ActionName and MachineId, and then pick the record with the highest ActionId for each group
Instead of grouping an ordered collection, group the collection first, and then select the record with the highest ID for each of the groups. GroupBy is not guaranteed to preserve the order in each group in LINQ to SQL - it depends on your database server.
var list = db.Actions.Where(z => z.RunId == RunId).GroupBy(c => new
{
c.ActionName,
c.MachineNumber,
})
.Select(y => y.OrderByDescending(z => z.ActionId).FirstOrDefault()).ToList();
This is my table. I want to get the distinct records as unique Name , SourceDeviceId, SourceState, Id . Since ID column consists of unique numbers I am getting all the records. But I want to get any of Id with Same Name, SourceDeviceId, SourceState. I have tried to use GroupBy but I am not able to select specific columns after select statement.
You can do this a few ways, however this might help
var result = db.SomeTable.GroupBy(x => new
{
x.Name,
x.SourceDeviceId
})
.Select(x => x.First())
.ToList();
It gives you back a list of distinct entities by Name and SerouceDeviceId
Or if you want an anonymouse type
var result = db.SomeTable.GroupBy(x => new
{
x.Name,
x.SourceDeviceId
})
.Select(x => x.Key)
.ToList();
Consider using Distinct:
var result = db.SomeTable.Select(x => new
{
x.Name,
x.SourceDeviceId
})
.Distinct()
.ToList();
The ToList is optional - whether you need it depends on how you plan to use result.
I have the following code in Linq, and I was wondering how to make it so that it groups all others beside the top 3 into an others category and sum their volumes.
var list = (from t in sortedCollection.DataItem
orderby t.volume
select t).Take(3);
You need to use Skip to ignore top 3 and group the rest like:
var list = (from t in sortedCollection.DataItem
orderby t.volume
select t).Skip(3);
From the comments, it seems you only want to get the sum of a particular field after skipping first 3 records.
var sum = (from t in sortedCollection.DataItem
orderby t.volume
select t).Skip(3).Sum(r=> r.VOLUME);
Or with a complete method syntax:
var Sum = sortedCollection.DateItem.OrderBy(t => t.volume)
.Skip(3)
.Sum(r=> r.volume);
If you need grouping , that it would look like:
With method syntax it should be something like:
var query = sortedCollection.DateItem.OrderBy(t => t.volume)
.Skip(3)
.GroupBy(t => t.YourGroupingField);
To do Sum based on a field you can do something like:
var query = sortedCollection.DateItem.OrderBy(t => t.volume)
.Skip(3)
.GroupBy(t => t.YourGroupingField)
.Select(grp => new SqlCommand(
{
Key = grp.Key,
Sum = grp.Sum(r=> r.ValueFieldForSum)
}));
I have a list of Objects, where every object has a "Name" and different other stuff. I want to filter out those objects who don't have a unique name in that List.
Is there a LINQ Statement where I can "Union()" all the resulting groups and just return a IEnumerable of my objects?
Like
IEnumerable<MyObject> Results = (from x in Objects
group x by x.Name into g
where g.Count() > 1
select g)
.COMBINE_OR_WHATEVER();
Thanks!
Yes, there is. You can use SelectMany.
IEnumerable<MyObject> Results = (from x in Objects
group x by x.Name into g
where g.Count() > 1
select g)
.SelectMany(x => x);
I think you want only the object with unique names ("I want to filter out those objects who don't have a unique name in that List"):
var result = Objects.GroupBy(o => o.Name)
.Where(grp => grp.Count() == 1)
.SelectMany(grp => grp);