I'm having the hardest time wrapping my mind around LINQ .
I'm querying database
date ColumnC
25-04-2016 10
01-05-2016 8
10-05-2016 4
That I need to Group and Sum by year and month.
I tried to do this but it doesn't work :
public TYPE getX(DateTime value) {
var total = from p in context.table
.Where(p => p.date.Year == value.Year)
group p by p.date.Year into dp
select new
{
result = dp.Sum(s => s.ColumnC),
};
return total;
}
i also could not know the return type.
in other words :
how to get a linq query from this :
select Month(date) Month ,sum(ColumnC) result
from table
group by(Month(date))
==============
Thanks in advance
If you want to have result you have to cast your sum explicitly, therefore losing some data. This is necessary because your initial data is decimal. Therefore the code:
var total = (from p in context.table
.Where(p => p.date.Year == value.Year)
group p by p.date.Month into dp
select new KeyValuePair<int,int> (dp.Key, (int)dp.Sum(s => s.ColumnC))
);
The result of this will be IQueryable<KeyValuePair<int,int>> which is one way to also know the type.
Second option which is more correct would be not to cast the decimal to integer, therefore the code:
var total = (from p in context.table
.Where(p => p.date.Year == value.Year)
group p by p.date.Month into dp
select new KeyValuePair<int,decimal> (dp.Key, dp.Sum(s => s.ColumnC))
);
The result will be IQueryable<KeyValuePair<int,decimal>> which is pretty easy to work with.
In both cases the Key is the Month and the Value is the Sum of ColumnC
Your return type here is partially anonymous, which means you cannot ever know the type.
You can, however, directly retrieve the result as part of the grouping:
public IEnumerable<IGrouping<int, decimal>> getTotalHeures(DateTime value)
{
var total = from p in context.table
.Where(p => p.date.Year == value.Year)
.GroupBy(p => p.date.Month, p => p.ColumnC);
return total;
}
Related
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.
var a = 123;
var query = (from p in a where p.ID == a) //gives 10 records.
.Sum(Convert.Int32.Parse(x => x.balance); // this line gives the following message.
Error Message.
LINQ to Entities does not recognize the method 'Int32 ToInt32 (System.String)',
Database design for the varchar column can't change to int or double.
var a = 123;
var sum = (from p in a where p.ID == a) //gives 10 records.
.Sum(x => int.Parse(x.balance));
var sum = (from p in db.table where p.ID == a)
.AsEnumerable() // #juharr's suggestion
.Sum(x => Convert.ToInt32(x.balance));
You want to convert the string in each iteration.. not the loop itself
This assumes the balance is a string and not an int already
If Entity Framework (or Linq-to-SQL, not sure what ORM are you using) can't translate something to SQL syntax, you can always try to resolve the query "C#-side" or client-side.
If you add .ToList() before the .Sum(), that should force the query to resolve on the database, and then the .Sum(...) and the .ToInt32() will be executed in memory.
var sum = (from p in db.table where p.ID == a)
.ToList()
.Sum(x => Convert.ToInt32(x.balance));
I am looking for a way in LINQ to get a max Id record by using 'Group By' clause
Consider the following Sample data
Table: ProcessAud
ProcessSeq ProjectSeq ProjectValue Active
11 1 50000 Y
12 1 10000 Y
13 2 70000 Y
14 2 90000 Y
In which I want to get two records as a list such that is second and fourth
records (i.e) ProcessSeq 12 and 14. And I tried it like following
var ProcessAudList = ProcessAudService.FilterBy(x => x.Active == "Y"
).GroupBy(x => x.ProjectSeq).Max().ToList();
It is not working properly, So how to do it in LINQ. Please anybody help.
You want to pick top record from each group.
var ProcessAudList = ProcessAudService.Where(x => x.Active == "Y")
.GroupBy(x => x.ProjectSeq, (key,g)=>g.OrderByDescending(e=>e.ProjectValue).First());
Check demo code
When you use GroupBy extension, method will return you IGrouping instance and you should query IGrouping instance like below;
var ProcessAudList = collection.Where(x => x.Active == "Y").GroupBy(x => x.ProjectSeq).Select(x => x.OrderByDescending(a => a.ProcessSeq).FirstOrDefault()).ToList();
Hope this helps
You're most of the way there, but Max is the wrong term to use.
Each IGrouping is an IEnumerable (or IQueryable) sequence of its own, so you can use OrderBy and First clauses to get the answer you need:
var ProcessAudList = ProcessAudService
.FilterBy(x => x.Active == "Y")
.GroupBy(x => x.ProjectSeq)
.Select(grp => grp.OrderByDescending(x => x.ProcessSeq).First())
.ToList();
The Select clause will process each of the groups, order the groups descending by ProcessSeq and select the first one. For the data you provided this will select the rows with ProcessSeq equal to 12 and 14.
With this code you can get all max id in foreach
var res = from pa in ProcessAud
group Cm by pa.ProjectSeq into Cm1
select new
{
_max = Cm1.Max(x => x.ProcessSeq)
};
foreach (var item in res)
{
//item._max have biggest id in group
}
I'm using LINQ on an IQueryable returned from NHibernate and I need to select the row with the maximum value(s) in a couple of fields.
I've simplified the bit that I'm sticking on. I need to select the one row from my table with the maximum value in one field.
var table = new Table { new Row(id: 1, status: 10), new Row(id: 2, status: 20) }
from u in table
group u by 1 into g
where u.Status == g.Max(u => u.Status)
select u
This is incorrect but I can't work out the right form.
BTW, what I'm actually trying to achieve is approximately this:
var clientAddress = this.repository.GetAll()
.GroupBy(a => a)
.SelectMany(
g =>
g.Where(
a =>
a.Reference == clientReference &&
a.Status == ClientStatus.Live &&
a.AddressReference == g.Max(x => x.AddressReference) &&
a.StartDate == g.Max(x => x.StartDate)))
.SingleOrDefault();
I started with the above lambda but I've been using LINQPad to try and work out the syntax for selecting the Max().
UPDATE
Removing the GroupBy was key.
var all = this.repository.GetAll();
var address = all
.Where(
a =>
a.Reference == clientReference &&
a.Status == ClientStatus.Live &&
a.StartDate == all.Max(x => x.StartDate) &&
a.AddressReference == all.Max(x => x.AddressReference))
.SingleOrDefault();
I don't see why you are grouping here.
Try this:
var maxValue = table.Max(x => x.Status)
var result = table.First(x => x.Status == maxValue);
An alternate approach that would iterate table only once would be this:
var result = table.OrderByDescending(x => x.Status).First();
This is helpful if table is an IEnumerable<T> that is not present in memory or that is calculated on the fly.
You can also do:
(from u in table
orderby u.Status descending
select u).Take(1);
You can group by status and select a row from the largest group:
table.GroupBy(r => r.Status).OrderByDescending(g => g.Key).First().First();
The first First() gets the first group (the set of rows with the largest status); the second First() gets the first row in that group.
If the status is always unqiue, you can replace the second First() with Single().
Addressing the first question, if you need to take several rows grouped by certain criteria with the other column with max value you can do something like this:
var query =
from u1 in table
join u2 in (
from u in table
group u by u.GroupId into g
select new { GroupId = g.Key, MaxStatus = g.Max(x => x.Status) }
) on new { u1.GroupId, u1.Status } equals new { u2.GroupId, Status = u2.MaxStatus}
select u1;
What about using Aggregate?
It's better than
Select max
Select by max value
since it only scans the array once.
var maxRow = table.Aggregate(
(a, b) => a.Status > b.Status ? a : b // whatever you need to compare
);
More one example:
Follow:
qryAux = (from q in qryAux where
q.OrdSeq == (from pp in Sessao.Query<NameTable>() where pp.FieldPk
== q.FieldPk select pp.OrdSeq).Max() select q);
Equals:
select t.* from nametable t where t.OrdSeq =
(select max(t2.OrdSeq) from nametable t2 where t2.FieldPk= t.FieldPk)
Simply in one line:
var result = table.First(x => x.Status == table.Max(y => y.Status));
Notice that there are two action.
the inner action is for finding the max value,
the outer action is for get the desired object.
I am trying to create a lambda expression (Linq, C# 3.5) that can perform a OrderBy on a value that is of data type String but which actually contains a parse-able DateTime.
For example, typical values may be "5/12/2009" , "1/14/2008", etc.
The OrderBy clause below works correctly for ordering (as if string data), but I actually want to treat the values as DateTimes, and perform the sort by Date. (The sortColumn would be something like "dateCreated".)
List<MyObject> orderedList = unorderedList.OrderBy(p => p.Details.Find(s => s.Name == sortColumn).Value).ToList();
Is there a way to convert the values in the predicate to do this? Any help appreciated!
Rather gross and inefficient:
List<MyObject> orderedList = unorderedList.OrderBy(p => DateTime.Parse(p.Details.Find(s => s.Name == sortColumn).Value)).ToList();
To reduce the number of lookups/parsing:
List<MyObject> orderedList =
(from extracted in (from p in unorderedList
select new { Item = p, Date = DateTime.Parse(p.Details.Find(s => s.Name == sortColumn).Value })
orderby extracted.Date
select extracted.Item)
.ToList();
Project the date/time value and then sort by it.
var orderedList =
(from p in unorderedList
let value = DateTime.Parse(p.Details.Find(s => s.Name == sortColumn).Value)
orderby value
select p)
.ToList();