How can you handle this sub-query with LINQ to SQL? - linq

Question

I'm a bit stuck on this. Basically I want to do something like the following SQL query in LINQ to SQL:
SELECT *
FROM UnitPrice
WHERE EffectiveDateTime = (SELECT MAX(EffectiveDateTime)
FROM UnitPrice AS InnerUnitPrice
WHERE InnerUnitPrice.EffectiveDateTime < GETDATE())

Answer 1

You could do the following.
Emulating your scenario with DataTable
var unitPrice = new DataTable();
unitPrice.Columns.Add("EffectiveDateTime",typeof(DateTime));
unitPrice.Columns.Add("SomeOther",typeof(string));
unitPrice.Rows.Add(new DateTime(2018,12,1), "Sample1");
unitPrice.Rows.Add(new DateTime(2018,12,2), "Sample2");
unitPrice.Rows.Add(new DateTime(2018,12,3), "Sample3");
unitPrice.Rows.Add(new DateTime(2018,12,4), "Sample41");
unitPrice.Rows.Add(new DateTime(2018,12,4), "Sample4");
unitPrice.Rows.Add(new DateTime(2019,12,4), "Sample5");
You can query the required result as,
var result = unitPrice.AsEnumerable()
.Where(x=>x.Field<DateTime>("EffectiveDateTime") < DateTime.Today)
.GroupBy(x=>x.Field<DateTime>("EffectiveDateTime"))
.OrderByDescending(x=>x.Key)
.First().ToList();
Output
04-12-2018 00:00:00 Sample41
04-12-2018 00:00:00 Sample4

Answer 2

I give you an example on this short list of objects
List < Entry > list = new List<Entry>();
list.Add(new Entry("a","_a",new DateTime(2019,1,30),1));
list.Add(new Entry("b", "_b", new DateTime(2018, 12, 31), 2));
list.Add(new Entry("c", "_c", new DateTime(2018, 12, 31), 3));
list.Add(new Entry("d", "_d", new DateTime(2018, 12, 30), 4));
list.Add(new Entry("e", "_e", new DateTime(2018, 11, 30), 5));
the properties of the class Entry are, in order Reference (string), Donation (string), Date (datetime), Amount (double).
First step, we want to select the most recent date which is before today (what you're doing in your subquery); starting from our list we could do something like this:
var max_date = from l in list
where l.Date < DateTime.Today
group l.Date by 1 into g
select new { Val=g.Max() };
foreach (var m in max_date)
{
Console.WriteLine(m.Val);
}
Running this you'll get 31/12/2018 as desired. But we need another step to select all the informations on the selected date. So, step 2, we put our starting list in inner join with the one we've just built:
var list2 = from l in list
join m in max_date on l.Date equals m.Val
select l;
foreach (var l in list2)
{
Console.WriteLine($"{l.Reference}, {l.Donation}, {l.Date}, {l.Amount}");
}
the result will be
b, _b, 31/12/2018 00:00:00, 2
c, _c, 31/12/2018 00:00:00, 3
as desired. Hope it helps!

Answer 3

Linq lamda query:
var q = db.UnitPrice
.Where(x1 => x1.EffectiveDateTime == db.UnitPrice
.Where(x2 => x2.EffectiveDateTime < DateTime.Now)
.Max(x3 => x3.EffectiveDateTime)
)
.ToList();