Linq query: MAX in WHERE - linq

Question

I have a long query that returns
Item {
DateTime entryDate
.....
}
I like to combine the result of this query with another table
Value {
DateTime date,
double value
}
such that if entryDate >= CUTOFF, then take the value on CUTOFF, else take the value on entryDate. In other words, I'd want to achieve:
SELECT Item.*, Value.value WHERE
MIN( Item.entryDate, CUTOFF ) == Value.date
Excuse my syntax, but that's the idea.
EDIT: After some trial and error, I came up with this linq-to-sql query:
from iValue in Values
join iItem in ... (long query)
let targetDate = iItem.EntryDate > CUTOFF ? iItem.EntryDate : CUTOFF
where iValue.Date == targetDate
select new
{
iItem,
targetDate,
iValue
}
Thanks for your help.

Answer 1

yourLongQuery.Where(y => y.Item.entryDate == Value.date || CUTOFF == Value.date)
.Select(x => new {
entrydate = (x.Item.entryDate < CUTOFF ? x.Item.entryDate : CUTOFF),
/*rest of x.Item properties here */ ,
x.Value.date,
x.Value.value
});
Filter the query, Combine the two items into one item and modify the first item

Answer 2

Given that you've returned your data from your Item query and that the Value table is relatively small then this is a nice way to go:
var lookup = values.ToLookup(v => v.date, v => v.value);
var query =
from i in items
let c = i.entryDate < CUTOFF ? i.entryDate : CUTOFF
let v = lookup[c].FirstOrDefault()
select new
{
Item = i,
Value = v,
};
The ToLookup extension is very useful and often overlooked.

Answer 3

such that if entryDate >= CUTOFF, then take the value on CUTOFF, else
take the value on entryDate. In other words, I'd want to achieve:
SELECT Item.*, Value.value WHERE MAX( Item.entryDate, CUTOFF ) ==
Value.date
This is contradictory - if entryDate >= CUTOFF then take the value on CUTOFF imples that you want MIN(Item.entryDate, CUTOFF), not MAX.
Having said that, you just want to select the value.Value that matches each item of your query. Each item should look up the relevant value which matches your MAX (or, I believe, MIN) statement.
query.Select(item =>
{
var matchingValue = Values.Single(v =>
v.date == Min(item.entryDate, CUTOFF));
return new { item, matchingValue.value };
});
This will return an IQueryable of anonymous { Item, double } objects.
If you require this to be a executed as a single SQL statement you'll need to do some refactoring. A good start is to swap matchingValue into a single statement.
query.Select(item => new { item, context.Values.Single(v =>
v.date == Min(item.entryDate, CUTOFF)).value });
I don't have a Visual Studio in front of me to confirm, but I am not sure that the Math.Min function is mapped in LINQ-to-SQL. Let's assume it's not.
query.Select(item => new { item, context.Values.Single(v =>
v.date == (item.entryDate < CUTOFF ? item.entryDate : CUTOFF)).value });
I believe that will resolve to a single query if you execute it with a .ToList() but can't confirm until I have some tools in front of me. Test it with SQL profiler to be sure.