I have tried for sum time now but I cant get the hold of doing this selection in LINQ to Entites? How can I do a Select on a Selection?
SELECT * FROM tProduct AS tpp, tProductGroupDetail AS tppg,
(SELECT tpgd.ProductGroupID AS pgID
FROM tProduct AS tp, tProductGroupDetail AS tpgd
WHERE tp.ProductID = 428 AND tpgd.ProductID = tp.ProductID) AS JoinedTable
WHERE JoinedTable.pgID = tppg.ProductGroupID
AND tpp.ProductID = tppg.ProductGroupID
Your SQL statement is very bizarre. Isn't it equivalent to the following statement?
SELECT *
FROM
tProduct AS p
INNER JOIN tProductGroupDetail AS pgd
ON p.ProductID = pgd.ProductGroupID
WHERE
pgd.ProductID = 428
The corresponding LINQ query would then be rather obvious.
Let's change your SQL step by step:
The nested SELECT looks like this:
(SELECT tpgd.ProductGroupID AS pgID
FROM tProduct AS tp, tProductGroupDetail AS tpgd
WHERE tp.ProductID = 428 AND tpgd.ProductID = tp.ProductID)
This is the same as
(SELECT tpgd.ProductGroupID AS pgID
FROM tProductGroupDetail AS tpgd
WHERE tpgd.ProductID = 428)
Since tpgd.ProductID = tp.ProductID you can directly directly test for tpgd.ProductID = 428 without having to join with tp. The simplyfied statement now looks like this:
SELECT * FROM tProduct AS tpp, tProductGroupDetail AS tppg,
(SELECT ProductGroupID
FROM tProductGroupDetail
WHERE ProductID = 428) AS JoinedTable
WHERE JoinedTable.ProductGroupID = tppg.ProductGroupID
AND tpp.ProductID = tppg.ProductGroupID
Basically, what it does, is to join tProduct with tProductGroupDetail. The purpose of the additional join with JoinedTable is to restrict the records to contain only product groups having a ProductID = 428. This condition can be applied directly:
SELECT *
FROM tProduct AS tpp, tProductGroupDetail AS tppg,
WHERE tppg.ProductID = 428 -- Note: ProductID of tggp, NOT tpp!
AND tpp.ProductID = tppg.ProductGroupID
The last step is to replace the join with an INNER JOIN clause and to tweak the aliases. This yields my first SQL statement.
To obtain the list of ProductGroup containing the product #428, just write:
dataContext.Products.FirstOrDefault(p => p.ProductID == 428).ProductGroups
This solution assumes the existence in the entity Product of the navigation property ProductGroups. If this is not the case, thank you to specify the entity model.
I think this should do it (although it's off the top of my head, not tested):
var product = Products.Where(x => x.ProductGroup.Any(y => y.ProductID == 428));
Related
We are currently trying to write out sorting into our server-side pagination using Linq and EF Core 2. We are running into an issue where the column alias being produced by Linq does not work while using pagination. However if we do not paginate it works as intended.
All of the columns within the outputted queries are aliases as we have different property names in the model and database column names are different, but this shouldn't make a difference to our knowledge.
This is the Linq query without the pagination:
var source = from p in _ppmRepository.GetAll()
join jt in _jobTypeRepository.GetAll() on p.PpmFkeyInSeq equals jt.Id into jtdata
from jt in jtdata.DefaultIfEmpty()
join a in _assetRepository.GetAll() on p.PpmFkeyArSeq equals a.Id into aData
from a in aData.DefaultIfEmpty()
where p.PpmFkeyBgSeq == bldId
orderby p.PpmFreq
select new BuildingPpmListViewModel
{
PpmId = p.Id,
PpmFreq = p.PpmFreq,
PpmNextService = p.PpmNextService,
TotalCost = p.TotalCost,
PpmPeriodUnits = p.PpmPeriodUnits,
PpmFkeyPriDesc = p.PpmFkeyPriDesc,
JtTitle = jt.JtTitle,
AssetId = p.PpmFkeyArSeq,
AssetDescription = a.AssetDescription,
IsDeleted = p.IsDeleted
};
source = source.Where(i => i.JtTitle.Contains("audit") && i.AssetDescription.Contains("df"));
This is the outputted query produced by ef core which works:
SELECT [p].[PPM_SEQ] AS [PpmId], [p].[PPM_FREQ] AS [PpmFreq], [p].[PPM_NEXT_SERVICE] AS [PpmNextService],
CAST([p].[TotalCost] AS float) AS [TotalCost], [p].[PPM_PERIOD_UNITS] AS [PpmPeriodUnits], [p].[PPM_FKEY_PRI_DESC] AS [PpmFkeyPriDesc],
[t].[jt_title] AS [JtTitle], [p].[PPM_FKEY_AR_SEQ] AS [AssetId], [t0].[AR_DESCRIPTION] AS [AssetDescription], [p].[Deleted] AS [IsDeleted]
FROM [PPMs] AS [p]
LEFT JOIN (
SELECT [j].*
FROM [JobTypes] AS [j]
) AS [t] ON [p].[PPM_FKEY_IN_SEQ] = [t].[jt_seq]
LEFT JOIN (
SELECT [a].*
FROM [Assets] AS [a]
) AS [t0] ON [p].[PPM_FKEY_AR_SEQ] = [t0].[ar_seq]
WHERE ([p].[PPM_FKEY_BG_SEQ] = 172) AND ((CHARINDEX(N'audit', [t].[jt_title]) > 0) AND (CHARINDEX(N'df', [t0].[AR_DESCRIPTION]) > 0))
ORDER BY [PpmFreq]
This is the Linq query with the pagination:
var source = from p in _ppmRepository.GetAll()
join jt in _jobTypeRepository.GetAll() on p.PpmFkeyInSeq equals jt.Id into jtdata
from jt in jtdata.DefaultIfEmpty()
join a in _assetRepository.GetAll() on p.PpmFkeyArSeq equals a.Id into aData
from a in aData.DefaultIfEmpty()
where p.PpmFkeyBgSeq == bldId
orderby p.PpmFreq
select new BuildingPpmListViewModel
{
PpmId = p.Id,
PpmFreq = p.PpmFreq,
PpmNextService = p.PpmNextService,
TotalCost = p.TotalCost,
PpmPeriodUnits = p.PpmPeriodUnits,
PpmFkeyPriDesc = p.PpmFkeyPriDesc,
JtTitle = jt.JtTitle,
AssetId = p.PpmFkeyArSeq,
AssetDescription = a.AssetDescription,
IsDeleted = p.IsDeleted
};
source = source.Where(i => i.JtTitle.Contains("audit") && i.AssetDescription.Contains("df")).Skip(0).Take(50);
This is the output of the pagination where in the over function order by PpmFreq is the alias of [p].[PPM_FREQ] that SQL can not find:
SELECT [t1].[PpmId], [t1].[PpmFreq], [t1].[PpmNextService], [t1].[TotalCost], [t1].[PpmPeriodUnits],
[t1].[PpmFkeyPriDesc], [t1].[JtTitle], [t1].[AssetId], [t1].[AssetDescription], [t1].[IsDeleted]
FROM (
SELECT [p].[PPM_SEQ] AS [PpmId], [p].[PPM_FREQ] AS [PpmFreq], [p].[PPM_NEXT_SERVICE] AS [PpmNextService],
CAST([p].[TotalCost] AS float) AS [TotalCost], [p].[PPM_PERIOD_UNITS] AS [PpmPeriodUnits], [p].[PPM_FKEY_PRI_DESC] AS
[PpmFkeyPriDesc], [t].[jt_title] AS [JtTitle], [p].[PPM_FKEY_AR_SEQ] AS [AssetId], [t0].[AR_DESCRIPTION] AS [AssetDescription],
[p].[Deleted] AS [IsDeleted], ROW_NUMBER() OVER(ORDER BY [PpmFreq]) AS [__RowNumber__]
FROM [PPMs] AS [p]
LEFT JOIN (
SELECT [j].*
FROM [JobTypes] AS [j]
) AS [t] ON [p].[PPM_FKEY_IN_SEQ] = [t].[jt_seq]
LEFT JOIN (
SELECT [a].*
FROM [Assets] AS [a]
) AS [t0] ON [p].[PPM_FKEY_AR_SEQ] = [t0].[ar_seq]
WHERE (([p].[PPM_FKEY_BG_SEQ] = 172)) AND ((CHARINDEX(N'audit', [t].[jt_title]) > 0)
AND (CHARINDEX(N'df', [t0].[AR_DESCRIPTION]) > 0))
) AS [t1]
WHERE ([t1].[__RowNumber__] > 0) AND ([t1].[__RowNumber__] <= (50))
This looks to be where our issues are coming from as we can slightly modify it to get a correct result from the database:
ROW_NUMBER() OVER(ORDER BY [PpmFreq]) AS [__RowNumber__]
If we were to modify the above statement to also include the table alias as [p].[PPM_FREQ], like so: ROW_NUMBER() OVER(ORDER BY [p].[PPM_FREQ]) AS [__RowNumber__] then our issues are resolved, but that doesnt seem possible with our current linq query.
See if following works better :
var source = (from p in _ppmRepository.GetAll()
join jt in _jobTypeRepository.GetAll() on p.PpmFkeyInSeq equals jt.Id into jtdata
from jt in jtdata.DefaultIfEmpty()
join a in _assetRepository.GetAll() on p.PpmFkeyArSeq equals a.Id into aData
from a in aData.DefaultIfEmpty()
select new BuildingPpmListViewModel
{
PpmId = p.Id,
PpBgSeq = p.PpmFkeyBgSeq,
PpmFreq = p.PpmFreq,
PpmNextService = p.PpmNextService,
TotalCost = p.TotalCost,
PpmPeriodUnits = p.PpmPeriodUnits,
PpmFkeyPriDesc = p.PpmFkeyPriDesc,
JtTitle = jt.JtTitle,
AssetId = p.PpmFkeyArSeq,
AssetDescription = a.AssetDescription,
IsDeleted = p.IsDeleted
})
.Where(x => x.PpBgSeq == bldId)
.OrderBy(x => x.PpmFreq)
.ToList();
This is a known issue of that we have later filed with the ef core team directly.
This is a known issue which has been fixed for upcoming release of 2.1
You can see more details and possible work-around here
github.com/aspnet/EntityFrameworkCore/issues/9535`
Smit Patel
If you run a nightly build you can fix the above issue.
I'm trying to convert the following MySQL statement in to LINQ query format
SELECT * FROM table1 WHERE table1.id IN (SELECT c_id FROM table2 WHERE a_id IN (1, 49) GROUP BY c_id HAVING COUNT(*) = 2) ORDER BY name
Got as far as this, but I'm drawing a blank on how to handle the IN and 2nd SELECT statement
myItems = from c in table1
let id = c.id
where ????
orderby c.name
select c;
Would appreciate some guidance with this please
Try this:
var ids=new[]{1,49};
var innerquery=table2.Where(e=>ids.Contains(e.a_id))
.GroupBy(e=>e.c_id)
.Where(g=>g.Count()==2)
.Select(g=>g.Key);
var myItems = from c in table1
where innerquery.Contains(c.id)
orderby c.name
select c;
First define your inner query,after the group by you will get a collection of IGrouping<TKey, TElement>> that represent a collection of objects that have a common key, filter the groups choosing only those where count==2, and select the keys of those groups. The second part is really easy to understand. I split the process in two queries to do it more readable, but you can merge both query in one.
I have to retrieve all the columns from table1 and matching columns from table2. I have a stored procedure as :
alter Procedure [dbo].[usp_Property]
#UserId bigint =null
As
Begin
select P.PID, P.PropertyName, P.SBUArea, P.ListedOn,
P.Availability, P.Price, F.UserID, F.PID as FavProjId
from dbo.Property P left outer join dbo.Favorite F
on (F.PID=P.PID And F.UserID=#UserId)
I want to get Linq query for the same. So far I tried with something like
//User Id comes from session..
//var userId
var result=(from p in Properties
join f in Favorites
on p.PID equals f.PID into r
from r1 in r.DefaultIfEmpty()
where r1.UserID==userId
select new
{
p.PID,
p.PropertyName,
p.SBUArea, p.ListedOn,
r1.UserId
});
Can anyone please correct me. I want to use left outer join or any other alternate thing here.
If I beautify your SP's code, I get this:
DECLARE #UserId int
SET #UserId = 12435
SELECT
P.PID
,P.PropertyName
,P.SBUArea
,P.ListedOn
,P.Availability
,P.Price
,F.UserID
,F.PID AS FavProjId
FROM Property AS P
LEFT JOIN Favorite AS F
ON (F.PID=P.PID AND F.UserID = #UserId)
Now I wonder if you need that UserId in the WHERE clause of the SQL, or really in the join.
But anyway, here the LINQ-equivalent of exactly that SQL:
System.Int64 __UserId = 12435;
var query = (
from P in Repo.Property
from F in Repo.Favorite
.Where(fav=> fav.PID == P.PID && fav.UserID == __UserId)
.DefaultIfEmpty() // <== makes join left join
select new
{
PID = P.PID
,PropertyName = P.PropertyName
,SBUArea = P.SBUArea
,ListenOn = P.ListedOn
,Availabiity = P.Availability
,Price = P.Price
,UserId = F.UserID
,FavProjId = F.PID
}
);
var data = (query).ToList();
Use anonymous objects in your selection
var result = from t in table1
join x in table2
on t.id equals x.id
select new { id = t.id, col1 = t.col1, col2 = x.col2 }
If you will put the where clause after join you may get null reference exception because DefaultIfEmpty returns default value for non matching rows. You can filter the records before joining itself like this:-
var result=(from p in Properties
join f in Favorites.Where(x => x.UserID == userId)
on p.PID equals f.PID into r
from r1 in r.DefaultIfEmpty()
select new
{
p.PID,
p.PropertyName,
p.SBUArea,
p.ListedOn,
r1.UserId
});
Please note you need to access properties of Favorites using r1.
Update:
As far as I have understood you need all records from Property table and only matching rows from Favorite table. But you have a filter on your Favorite table so the ultimate data source will differ. Let me make my point clear by this example:-
Suppose you have following data in Property table:-
PID PropertyName Availability Price
1 aaa true 20
2 bbb false 10
3 ccc true 50
4 ddd false 80
5 eee true 55
6 fff false 70
and Favorite table like this:-
FID PID UserId
1 4 1001
2 2 1005
3 5 1007
And let's say you want all records for UserId 1005, then the result should contain all the property Id's from 1 till 6 even if UserId 1005 doesn't match for property Id's 4 & 2 right? So the query above is as per this understanding. Check this Fiddle with same example and output.
So I have a many to many relationship between something known as Specialism and SpecialismCombo. What I'm trying to do is take an int[] of ids and check if there is a combo already that contains the specialisms with those ids.
I was close but not quite right.
Say I have specialisms with Ids 1 and 3 and I create a combo with those specialisms.
If I pass in 3 & 1 then it returns the expected combo id.
If I pass in 1 then it returns the combo id that has both 1 and 3.
I can't just rely on total number of specialisms associated with the combo. Because if a combo has two items, 1 and 4 and the items being matched on are 1 and 3 I don't want this coming back as a matched combo.
So it's like I do need the count of this result, and match the count of total specialisms associated to the combo. I don't quite get whether I'm after a subquery or detatchedcriteria or how to get the result I want using nhibernate criteria. Thanks for your help!
int[] SpecialismIds = ArrayExtensions.ConvertArray<int>(idCollection.Split(new char[] { '|' }));
ICriteria query = m_SpecialismComboRepository.QueryAlias("sc");
query.CreateAlias("sc.Specialisms", "s", NHibernate.SqlCommand.JoinType.InnerJoin);
ICriterion lastCriteria = null;
foreach(int i in SpecialismIds)
{
ICriterion currentCriteria = Restrictions.Eq("s.SpecialismId", i);
if (lastCriteria != null)
lastCriteria = Restrictions.Or(lastCriteria, currentCriteria);
else
lastCriteria = currentCriteria;
}
if (lastCriteria != null)
query.Add(lastCriteria);
IProjection IdCount = Projections.Count("s.SpecialismId").As("IdCount");
query.SetProjection(
Projections.GroupProperty("sc.SpecialismComboId"),
IdCount
);
query.Add(Restrictions.Eq(IdCount, SpecialismIds.Count()));
var comboId = query.List();
The sql being generated is:
SELECT this_.SpecialismComboId as y0_, count(s1_.SpecialismId) as y1_
FROM dbo.SpecialismCombo this_
inner join SpecialismComboSpecialism specialism3_ on this_.SpecialismComboId=specialism3_.SpecialismId
inner join dbo.Specialism s1_ on specialism3_.SpecialismComboId=s1_.SpecialismId WHERE s1_.SpecialismId = #p0
GROUP BY this_.SpecialismComboId HAVING count(s1_.SpecialismId) = #p1',N'#p0 int,#p1 int',#p0=3,#p1=1
EDIT - It seems like I either need the having to be something like...
HAVING count(s1_.SpecialismId) = (select count(SpecialismId)
from specialismComboSpecialism
where SpecialismComboId = y0
group by SpecialismComboId) == #p2
Or maybe it's simpler than that and I need to exclude SpecalismCombos where the combo.specialisms are not in the collection of ids.
Ie. if the combo has specialisms 1 and 3 but the collection only has 1.. then we could exclude this combo based on 3 not being in the collection…
Edit 8/8/2011
Went back to focusing on how to get the result I needed in SQL - and I believe this query works.
WITH CustomQuery AS
(
SELECT sc.SpecialismComboId,
count(s.SpecialismId) AS ItemCount
FROM SpecialismCombo sc
inner join SpecialismComboSpecialism scs on sc.SpecialismComboId = scs.SpecialismComboId
inner join Specialism s on s.SpecialismId = scs.SpecialismId
GROUP BY sc.SpecialismComboId
HAVING count(s.SpecialismId) = 2
)
SELECT CustomQuery.SpecialismComboId FROM CustomQuery
INNER JOIN SpecialismComboSpecialism scs on CustomQuery.SpecialismComboId = scs.SpecialismComboId
WHERE scs.SpecialismId in (1,4)
GROUP BY CustomQuery.SpecialismComboId
HAVING count(scs.SpecialismId) = 2
So now I just need to figure out how to call this procedure from my nhibernate code passing in the appropriate values :)
I also discovered in the process that my mapping class was wrong - as it was putting the wrong values in the mapping table (ie. the specialismid was ending up in the specialismcomboid field !)
Your solution should actually work well. The specialisms are filtered by id and there shouldn't be anything left that is not searched for, so count should work. Unless you have the same specialism joined more the once. This currentCriteria lastCriteria stuff looks a bit strange, may be there is an error. Just use Expression.In or Conjunction.
IProjection IdCount = Projections.Count("s.SpecialismId").As("IdCount");
IQuery query = session
.CreateCriteria<SpecialismCombo>("sc")
.CreateCriteria("Specialism", "s");
.Add(Expression.In("s.SpecialismId", SpecialismIds));
.SetProjection(
Projections.GroupProperty("sc.SpecialismComboId"),
IdCount);
.Add(Restrictions.Eq(IdCount, SpecialismIds.Count()));
Should result in a query like this:
select ...
from
SpecialismCombo sc
inner join -- linktable ...
inner join Specialism s on ...
where
s.SpecialismId in (1, 4)
Group By sc.SpecialismComboId
having count(*) = 2
The same in HQL
from SpecialismCombo sc
join sc.Specialism s
where s.id in (:ids)
group by sc
having count(*) = :numberOfIds
You could also join the specialism as many times as you have ids to find:
IQuery query = session.CreateCriteria<SpecialismCombo>("sc")
int counter = 0;
foreach(int id in ids)
{
string alias = "s" + counter++;
query
.CreateCriteria("Specialism", alias);
.Add(Expression.Eq(alias + ".SpecialismId", id));
}
should create a query like this:
select ...
from
SpecialismCombo sc
inner join -- linktable ...
inner join Specialism s0 on ...
inner join -- linktable ...
inner join Specialism s1 on ...
where
s0.SpecialismId = 1
and s1.SpecialismId = 4
So I ended up creating a stored proc and using SQL CTE in order to get only the specialism combos with the correct count of specialisms. Posting this in case someone else comes across a similar issue.
Rediscovered after 8 months of using nhibernate that I'd forgotten a lot of SQL stuff :)
DECLARE #IdCollectionCount INT
, #IdCollection VARCHAR(250)
, #CollectionDelimiter NVARCHAR
SET #IdCollectionCount = 2;
SET #IdCollection = '1,4';
SET #CollectionDelimiter= ',';
WITH CustomQuery AS
(
SELECT sc.SpecialismComboId,
count(s.SpecialismId) AS ItemCount
FROM SpecialismCombo sc
inner join SpecialismComboSpecialism scs on sc.SpecialismComboId = scs.SpecialismComboId
inner join Specialism s on s.SpecialismId = scs.SpecialismId
GROUP BY sc.SpecialismComboId
HAVING count(s.SpecialismId) = #IdCollectionCount
)
SELECT Top 1 CustomQuery.SpecialismComboId FROM CustomQuery
INNER JOIN SpecialismComboSpecialism scs on CustomQuery.SpecialismComboId = scs.SpecialismComboId
INNER JOIN dbo.fn_SplitDelimited(#IdCollection,#CollectionDelimiter) AS ids
ON scs.SpecialismId = CAST(ids.ListValue AS INT)
GROUP BY CustomQuery.SpecialismComboId
HAVING count(scs.SpecialismId) = #IdCollectionCount
I have a left outer join (below) returning results as expected. I need to limit the results from the 'right' table to the 'first' hit. Can I do that somehow? Currently, I get a result for every record in both tables, I only want to see one result from the table on the left (items) no matter how many results I have in the right table (photos).
var query = from i in db.items
join p in db.photos
on i.id equals p.item_id into tempPhoto
from tp in tempPhoto.DefaultIfEmpty()
orderby i.date descending
select new
{
itemName = i.name,
itemID = i.id,
id = i.id,
photoID = tp.PhotoID.ToString()
};
GridView1.DataSource = query;
GridView1.DataBind();
This will do the job for you.
from i in db.items
let p = db.photos.Where(p2 => i.id == p2.item_id).FirstOrDefault()
orderby i.date descending
select new
{
itemName = i.name,
itemID = i.id,
id = i.id,
photoID = p == null ? null : p.PhotoID.ToString();
}
I got this sql when I generated it against my own model (and without the name and second id columns in the projection).
SELECT [t0].[Id] AS [Id], CONVERT(NVarChar,(
SELECT [t2].[PhotoId]
FROM (
SELECT TOP (1) [t1].[PhotoId]
FROM [dbo].[Photos] AS [t1]
WHERE [t1].[Item_Id] = ([t0].[Id])
) AS [t2]
)) AS [PhotoId]
FROM [dbo].[Items] AS [t0]
ORDER BY [t0].[Id] DESC
When I asked for the plan, it showed that the subquery is implemented by this join:
<RelOp LogicalOp="Left Outer Join" PhysicalOp="Nested Loops">
What you want to do is group the table. The best way to do this is:
var query = from i in db.items
join p in (from p in db.photos
group p by p.item_id into gp
where gp.Count() > 0
select new { item_id = g.Key, Photo = g.First() })
on i.id equals p.item_id into tempPhoto
from tp in tempPhoto.DefaultIfEmpty()
orderby i.date descending
select new
{
itemName = i.name,
itemID = i.id,
id = i.id,
photoID = tp.Photo.PhotoID.ToString()
};
Edit: This is Amy B speaking. I'm only doing this because Nick asked me to. Nick, please modify or remove this section as you feel is appropriate.
The SQL generated is quite large. The int 0 (to be compared with the count) is passed in via parameter.
SELECT [t0].X AS [id], CONVERT(NVarChar(MAX),(
SELECT [t6].Y
FROM (
SELECT TOP (1) [t5].Y
FROM [dbo].[Photos] AS [t5]
WHERE (([t4].Y IS NULL) AND ([t5].Y IS NULL)) OR (([t4].Y IS NOT NULL) AND ([t5].Y IS NOT NULL) AND ([t4].Y = [t5].Y))
) AS [t6]
)) AS [PhotoId]
FROM [dbo].[Items] AS [t0]
CROSS APPLY ((
SELECT NULL AS [EMPTY]
) AS [t1]
OUTER APPLY (
SELECT [t3].Y
FROM (
SELECT COUNT(*) AS [value], [t2].Y
FROM [dbo].[Photos] AS [t2]
GROUP BY [t2].Y
) AS [t3]
WHERE (([t0].X) = [t3].Y) AND ([t3].[value] > #p0)
) AS [t4])
ORDER BY [t0].Z DESC
The execution plan reveals three left joins. At least one is trivial and should not be counted (it brings in the zero). There is enough complexity here that I cannot clearly point to any problem for efficiency. It might run great.
You could do something like:
var q = from c in
(from s in args
select s).First()
select c;
Around the last part of the query. Not sure if it will work or what kind of wack SQL it will produce :)
Use an inner query. Include DefaultIfEmpty for the case of no photo and orderby for the case of more than one. The following example takes the photo with the greatest id.
var query =
from i in db.items
let p = from p in db.photos where i.id == p.item_id orderby p.id select p).DefaultIfEmpty().Last()
orderby i.date descending
select new {
itemName = i.name,
itemID = i.id,
id = i.id,
photoID = p.PhotoID
};
If you need to handle the case of no photo specially, you can omit DefaultIfEmpty and use FirstOrDefault/LastOrDefault instead.