Linq query to select rows where a column is a max value - linq

Question

I'd like to query a database table that looks like the simplified example below:
Quote | Sequence | Item
-------|-----------|-----
1 | 1.0M | a
1 | 2.0M | a
1 | 3.0M | a
1 | 1.0M | b
1 | 2.0M | b
1 | 3.0M | b
2 | 1.0M | x
2 | 2.0M | x
3 | 1.0M | y
and I need a query that gets all rows for a given Quote where the Sequence is the max value for that column:
Quote | Sequence | Item
-------|-----------|-----
1 | 3.0M | a
1 | 3.0M | b
2 | 2.0M | x
3 | 1.0M | y
I'm using F# and System.Data.Linq.
I can use
let quoteQuery =
query{
for row in db.[TABLE] do
select row
}
to get all rows, but I don't know Linq well enough--yet--to modify this to have the query that will produce the desired results. I've tried using the answer from this question in an attempt to modify my query, but I've hit a wall in trying to modify (guess?) the syntax/language necessary.
There are several SQL examples I can find, but few that are Linq-specific.

Answer 1

As are hinted in comments, this is not Linq, but f# query expressions.
And that is in fact not really what this question is about after all.
Its more set and relational algebra. Or something...
That said: The thing here is that if you group by and then get the max element of each group then you are good to good. Mind that the example code does not work against any DB or otherwise, but that should be rather easily replaceable.
type Table =
{
Quote:int
Sequence: decimal
Item: string
}
let createTableEntry (q,s,i) =
{
Quote = q
Sequence = s
Item = i
}
let printTR {Quote=q;Sequence=s;Item=i} = printfn "%A | %A | %A" q s i
let table =
[
(1 , 1.0M , "a")
(1 , 2.0M , "a")
(1 , 3.0M , "a")
(1 , 1.0M , "b")
(1 , 2.0M , "b")
(1 , 3.0M , "b")
(2 , 1.0M , "x")
(2 , 2.0M , "x")
(3 , 1.0M , "y")
]
|> List.map createTableEntry
let result =
table
|> List.groupBy (fun x -> x.Quote, x.Item) //group "unique" by Quote&Item
|> List.map (fun x -> snd x |> List.max) //get max of each group, i.e. max of Sequence
result |> Seq.iter printTR
1 | 3.0M | "a"
1 | 3.0M | "b"
2 | 2.0M | "x"
3 | 1.0M | "y"
Addendum
after Ivan Stoev had answered partially "wrongly". Here is a "corrected" version, which does the "same" (not really same, but ...) as the above:
let quoteQuery =
query {
for row in table do
groupBy (row.Quote, row.Item) into g
let maxRow =
query {
for row in g do
sortBy row.Sequence
headOrDefault
}
select maxRow
}
quoteQuery |> Seq.iter printTR
Addendum II
Since I edited and said that Ivans answer was not really the same as the first code example, I have also added one that is "exactly the same" with query expressions:
let quoteQuery' =
query {
for row in table do
groupBy (row.Quote, row.Item) into g
let maxRow =
query {
for row in g do
maxBy row.Sequence
}
select (fst g.Key, maxRow, snd g.Key)
}|>Seq.map createTableEntry
quoteQuery' |> Seq.iter printTR

Answer 2

F# Query expression are called query expression because they follow (losely) LINQ query syntax (vs method syntax) and they do implement Linq.IQueriable. So I also think about them as LINQ. While it maybe not idiomatic the above query can be rewritten in method syntax. If you nuget MoreLinq, it's quite succint. "Stealing" the above table definition from #HelgeReneUrholm:
open MoreLinq
table
.GroupBy(fun x -> (x.Quote,x.Item))
.Select(fun x -> x.MaxBy(fun x -> x.Sequence)) |> Seq.iter printTR
1 | 3.0M | "a"
1 | 3.0M | "b"
2 | 2.0M | "x"
3 | 1.0M | "y"