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Query with Linq and count matches in the Where clause and project it

Need to query many columns for possible matches and then see how many did match and add that to a column in the Select projection. I could have two column of the four match but how many did? Then want to sort the result by how many matches. Do I need to group them first? Do a separate aggregation? Kind of stumped as which way to go because the real one would have a lot more tests of fields in production. Could possibly match as many as 8 tests in the where clause.
var results = _RList
.Where(d => d.RMI15Min == RMI.ConfirmedBottom || d.RMI15Min == RMI.InPlaceBottomConfirmed
|| d.RMI30Min == RMI.ConfirmedBottom || d.RMI30Min == RMI.InPlaceBottomConfirmed)
.OrderBy(d => d.Instrument)
.Select(d => new
{
d.Instrument,
d.Description,
d.RMI15Min,
d.RMI30Min,
NewColumn with the total of the matches in the .Where clause above.
}).ToList();

Assuming _RList does not tie back to a database table
var confirmedList = new List<int> { RMI.ConfirmedBottom, RMI.InPlaceBottomConfirmed };
var results = _RList
.OrderBy(d => d.Instrument)
.Select(d => new
{
d.Instrument,
d.Description,
d.RMI15Min,
d.RMI30Min,
Count = (new List<int> { d.RMI15Min, d.RMI30Min }).Count(c => confirmedList.Contains(c))
})
.Where(d => d.Count > 0)
.ToList();
If it ties back to a database table, it depends on whether your library can convert the above LINQ statement.

I would consider using ternary operators to add 1/0 to the total;
Count = (d.RMI15Min == RMI.ConfirmedBottom ? 1 : 0)
+ (d.RMI15Min == RMI.InPlaceBottomConfirmed ? 1 : 0)
+ (d.RMI30Min == RMI.ConfirmedBottom ? 1 : 0)
+ (d.RMI30Min == RMI.InPlaceBottomConfirmed ? 1 : 0)
Then filter the list after calculating the count.

Getting the count of most repeated records in Linq

I am working on an application in which I have to store play history of a song in the data table. I have a table named PlayHistory which has four columns.
Id | SoundRecordingId(FK) | UserId(FK) | DateTime
Now i have to implement a query that will return the songs that are in trending phase i.e. being mostly played. I have written the following query in sql server that returns me data somehow closer to what I want.
select COUNT(*) as High,SoundRecordingId
from PlayHistory
where DateTime >= GETDATE()-30
group by SoundRecordingId
Having COUNT(*) > 1
order by SoundRecordingId desc
It returned me following data:
High SoundRecordingId
2 5
2 3
Which means Song with Ids 5 and 3 were played the most number of times i.e.2
How can I implement this through Linq in c#.
I have done this so far:
DateTime d = DateTime.Now;
var monthBefore = d.AddMonths(-1);
var list =
_db.PlayHistories
.OrderByDescending(x=>x.SoundRecordingId)
.Where(t => t.DateTime >= monthBefore)
.GroupBy(x=>x.SoundRecordingId)
.Take(20)
.ToList();
It returns me list of whole table with the count of SoundRecording objects but i want just count of the most repeated records.
Thanks

There is an overload of the .GroupBy method which will solve your problem.
DateTime d = DateTime.Now;
var monthBefore = d.AddMonths(-1);
var list =
_db.PlayHistories
.OrderByDescending(x=>x.SoundRecordingId)
.Where(t => t.DateTime >= monthBefore)
.GroupBy(x=>x.SoundRecordingId, (key,values) => new {SoundRecordingID=key, High=values.count()})
.Take(20)
.ToList();
I have simply added the result selector to the GroupBy method call here which does the same transformation you have written in your SQL.
The method overload in question is documented here
To go further into your problem, you will probably want to do another OrderByDescending to get your results in popularity order. To match the SQL statement you also have to filter for only counts > 1.
DateTime d = DateTime.Now;
var monthBefore = d.AddMonths(-1);
var list =
_db.PlayHistories
.Where(t => t.DateTime >= monthBefore)
.GroupBy(x=>x.SoundRecordingId, (key,values) => new {SoundRecordingID=key, High=values.count()})
.Where(x=>x.High>1)
.OrderByDescending(x=>x.High)
.ToList();

I like the 'linq' syntax it's similar to SQL
var query = from history in _db.PlayHistories
where history.DateTime >= monthBefore
group history by history.SoundRecordingId into historyGroup
where historyGroup.Count() > 1
orderby historyGroup.Key
select new { High = historyGroup.Count(), SoundRecordingId = historyGroup.Key };
var data = query.Take(20).ToList();

You´re allmost done. Just order your list by the count and take the first:
var max =
_db.PlayHistories
.OrderByDescending(x=>x.SoundRecordingId)
.Where(t => t.DateTime >= monthBefore)
.GroupBy(x=>x.SoundRecordingId)
.OrderByDescending(x => x.Count())
.First();
This gives you a single key-value-pair where the Key is your SoundRecordingId and the value is the number of its occurences in your input-list.
EDIT: To get all records with that amount chose this instead:
var grouped =
_db.PlayHistories
.OrderByDescending(x => x.SoundRecordingId)
.Where(t => t.DateTime >= monthBefore)
.GroupBy(x => x.SoundRecordingId)
.Select(x => new { Id = x.Key, Count = x.Count() }
.OrderByDescending(x => x.Count)
.ToList();
var maxCount = grouped.First().Count;
var result = grouped.Where(x => x.Count == maxCount);

This solves the problem by giving you what you asked for. Your query in LINQ, returning just the play counts.
var list = _db.PlayHistories.Where(x => x.DateTimeProp > (DateTime.Now).AddMonths(-1))
.OrderByDescending(y => y.SoundRecordingId.Count())
.ThenBy(z => z.SoundRecordingId)
.Select(xx => xx.SoundRecordingId).Take(20).ToList();

Determine Duplicate data using LINQ to EF

I have a dataset that i want to groupby to determine duplicate data.
Example i have a dataset that looks like this.
|id | Number | ContactID
1 1234 5
2 9873 6
3 1234 7
4 9873 6
Now i want to select data that has more than one occurrence of Number but only if the ContactID is not the same.
So basically return
| Number | Count |
1234 2
Any help would be appreciated using LINQ to EF, thanks.

Update:
All thanks to #DrCopyPaste, as he told me that I misunderstood your problem. Here is the correct solution:-
var result = from c in db.list
group c by c.Number into g
let count = g.GroupBy(x => x.ContactID).Where(x => x.Count() == 1).Count()
where count != 0
select new
{
Number = g.Key,
Count = count
};
Sample Fiddle.

This query avoids making a custom IEqualityComparer as if I remember correctly don't think they play well with EF.
var results = data.GroupBy(number => number.Number)
.Where(number => number.Count() > 1)
.Select(number => new
{
Number = number.Key,
Count = number.GroupBy(contactId => contactId.ContactId).Count(x => x.Count() == 1)
})
.Where(x => x.Count > 0).ToList();
Fiddle
It does an initial GroupBy to get all Numbers that are duplicated. It then selects a new type that contains the number and a second GroupBy that groups by ContactId then counts all groups with exactly one entry. Then it takes all results whose count is greater than zero.
Have not testing it against EF, but the query uses only standard Linq operators so EF shouldn't have any issues translating it.

Another way of doing this(using 1 level of grouping):
var results = data
.Where(x => data.Any(y => y.Id != x.Id && y.Number == x.Number && y.ContactId != x.ContactId))
.GroupBy(x => x.Number)
.Select(grp => new { Number = grp.Key, Count = grp.Count() })
.ToList();
Fiddle

Filter some unique Data with LINQ and C#

i am very new with C# and MVC.
My Problem:
I have a list OF IDs
int[] mylist = {10, 23}
I try to query some data from DB
var result = db.tableName.Where(o => mylist.Any(y => y == o.item_ID && o.readed)).ToList();
This is what I get with the query:
item_ID Product_ID readed
277 1232 1
277 1233 1
277 1235 1
280 1235 1
What I need is:
item_ID Product_ID readed
277 1235 1
280 1235 1
If I change "any" to "all" i don't get any results, but I have definitely one item where the condition fits.
I think its more like make a query with id 277, then a query with 280 and then merge the list and return only where where "Product_ID" match.
Any ideas?

I assume that what you need is this:
var temp = db.tableName.Where(o => mylist.Any(y => y == o.item_ID && o.readed))
.ToList();
// Find the Product_id which appeared more than one time
// By the way, this assumes that there is at least one product_Id whihc has appeared more than one time
var neededProductID = temp.GroupBy(x => x.Product_ID)
.Where(x => x.Count() > 1)
.First()
.Key;
// Filter the result by neededProductID
var result = temp.Where(x => x.Product_ID == neededProductID).ToList();
Also, if there could be more tha one Product_ID which has apperaed more than one time, then you can consider this:
var neededProductID = temp.GroupBy(x => x.Product_ID)
.Where(x => x.Count() > 1)
.Select(x => x.Key)
.ToList();
var result = temp.Where(x => neededProductID.Any(y => y == x.Product_ID)).ToList();
By the way, you don't need All(). It tells you if all the elements in a collection match a certain condition.

You can use the following
var result = db.tableName.Where(o => mylist.conains(o.item_ID)
&& o.readed).ToList();

LINQ Using Max() to select a single row

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.