linq query, search by max date if previous date exists - linq

Question

I'm trying to get three columns back from the database, DecalExpireDate, DecalExpireMonth and DecalExpireYear. Basically, I'm trying to check if it has a previous expiration date and if it does, I need to get the latest expiration date, as it may have multiple previous expiration dates.
var previousExpirationDate = (from d in db.CT_Decals
where d.TankID == decal.TankID
&& d.DecalStatus == "Approved"
&& d.DecalExpireDate == ((from dn in db.CT_Decals
where dn.TankID == decal.TankID
&& dn.DecalStatus == "Approved"
select dn.DecalExpireDate).Max())
select new
{
d.DecalExpireDate,
d.DecalExpireMonth,
d.DecalExpireYear
});
This query isn't working, can anybody see the problem? I'm searching by TankID, Status = approved and then I try to use the max expirationDate.

Answer 1

How about this
var result = db.CT_Decals.Where(o => o.TankId == decal.TankId && o.DecalStatus == "Approved")
.OrderByDescending(o => o.DecalExpireDate)
.Select(o => new { o.DecalExpireDate,
o.DecalExireMonth,
o.DecalExireYear }).First()
or
var result = (from d in db.CT_Decals
where d.TankID == 1 && d.Status == "Approved"
orderby d.ExpireDate descending
select new { d.DecalExpireDate,
d.DecalExpireMonth,
d.ExpireDate }).First();

Answer 2

The problem it's with the that max function couldn't be translated to Sql in the scope that you need so you could try like this
var previousExpirationDate = (from d in db.CT_Decals
where d.TankID == decal.TankID
&& d.DecalStatus == "Approved"
&& d.DecalExpireDate == ((from dn in db.CT_Decals
where dn.TankID == decal.TankID
&& dn.DecalStatus == "Approved"
order by dn.DecalExpireDate
select dn.DecalExpireDate).First())
select new
{
d.DecalExpireDate,
d.DecalExpireMonth,
d.DecalExpireYear
});

Answer 3

Although Max may be the problem in your example, it would be nice to find a way to use it properly because it is faster than sorting. How about:
Edit: this answer only returns the DecalExpireDate
var previousExpirationDate = (from d in db.CT_Decals
where d.TankID == decal.TankID
&& d.DecalStatus == "Approved"
select new
{
d.DecalExpireDate,
d.DecalExpireMonth,
d.DecalExpireYear
}).Max(d => d.DecalExpireDate);
For this version you will have to test to see if it's worth making an extra call to the database to avoid sorting:
var latestDate = (from d in db.CT_Decals
where d.TankID == decal.TankID
&& d.DecalStatus == "Approved"
select d.DecalExpireDate).Max();
var previousExpirationDate = (from d in db.CT_Decals
where d.TankID == decal.TankID
&& d.DecalStatus == "Approved"
&& d.DecalExpireDate == latestDate
select new
{
d.DecalExpireDate,
d.DecalExpireMonth,
d.DecalExpireYear
}).First();