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Fluent LINQ query with multiple join conditions, one of which being a simple equals and another a less than comparison between ids

I have a SQL query which includes a left join and a group by- so far so good- my trouble arises from one of the join conditions not being a straight "equals to" and I'm lost where to go with LINQ.
I know multiple join conditions usually involves creating a couple of anonymous objects and comparing them, but when I add an "equal to" and "a greater" than into the mix, I've no idea how that applies.
Here's what I'd like the query to look like if I had invented LINQ, but I know the "and" in my join condition is invalid;
var query =
from csp in db.ChatSessionPersons
join cm in db.ChatMessages on
csp.ChatSessionId equals cm.ChatSessionId
and cm.Id > csp.LastReadChatMessageId
// (i know these should be the other way round,
// but for readability I present them like this!)
into j1
from j2 in j1.DefaultIfEmpty()
group j2 by csp.ChatSessionId into grouped
select new {
ChatSessionId = grouped.Key,
UnreadCount = grouped.Count(t => t.Id != null)};
Any ideas anyone?

You can convert the non-equality condition to a lambda Where on the group join result.
var query = from csp in db.ChatSessionPersons
join cm in db.ChatMessages on csp.ChatSessionId equals cm.ChatSessionId into cmj
select new {
ChatSessionId = csp.ChatSessionId,
UnreadCount = cmj.Where(cm => cm.Id > csp.LastReadChatMessageId).Count()
};
NOTE: I modified the query a bit to remove the group by which isn't needed if you are using a group join that has already grouped the matching results, and to remove the left join DefaultIfEmpty which also isn't needed when processing a group join with something like Count, unless you wanted to return an UnreadCount of 1 when there are no matches, in which case you should put DefaultIfEmpty() before Count().
Of course, you could use query comprehension in the sub-query:
var query = from csp in db.ChatSessionPersons
join cm in db.ChatMessages on csp.ChatSessionId equals cm.ChatSessionId into cmj
select new {
ChatSessionId = csp.ChatSessionId,
UnreadCount = (from cm in cmj where cm.Id > csp.LastReadChatMessageId select cm).Count()
};

LINQ Left Join SQL equivalent via DefaultIfEmpty fails to return results

I have had an extensive look around on SE, tried all of the suggestions, checked out MSDN how to perform Left Join equivalent in LINQ to SQL and I have constructed my LINQ query according to MSDN example.
However, the result is not what SQL would return and I am completely lost as to where am I going wrong.
Here is some details:
I have two tables, Customers and Reports. A customer can submit many reports or none. In the current state I have many more reports than customers.
LINQ code:
var query = {from c in customers
join r in reports on c.Id equals r.Id into temp
from items in temp.DefaultIfEmpty()
select new {
c.Id,
LastReportDate = items?.DateCreated ?? DateTime.MinValue
}).ToList();
SQL code:
SELECT [Customers].[Id], R.LastReport AS LastReportDate FROM [Customers]
LEFT JOIN (
SELECT Reports.Id, MAX( [Reports].[Created] ) AS LastReport
FROM Reports GROUP BY Reports.Id
) AS r ON [Customers].[Id] = r.[Id]
The problem is that the query returns number of elements equal to number of reports. However, what I want is to get a list with all customers and for those who have submitted a report I wish to display the date of the most recent report, for those who have not submitted anything, I am happy to leave it NULL or DateTime.MinValue
Any help would be greatly appreciated. I guess I am missing a group by call somewhere in my LINQ code...

Im thinking probably something like this:
var query =
from c in customers
join r in reports on c.Id equals r.Id into g
select new
{
c.Id,
LastReportDate = g.Max(x => (DateTime?)x.Created)
};

you are now joining on join r in reports on c.Id equals r.Id into temp
this looks like: join on a customer.Id on Reports.Id, since you say there are 1 to many relation/rapport. I think your table will have a Reports.CustomerId. Is this correct?
So your query should something look like:
var results = customer.Where(c => c.Reports.Any())
.SelectMany(c => {c, c.Reports.Max(r => r.Created)})
.ToList();
the select comes out of my head, so i am probably missing something ;)
Have you tried LinqPad ? There you can type your linq-queries, and directly see your sql code and results. Works like a charm!

How to convert Group By SQL to LINQ?

I have the following SQL statement I'm trying to convert to Entity Framework.
SELECT S_NUMBER,A_NUMBER,FIRST_NAME,LAST_NAME
FROM EMPLOYEE WHERE S_NUMBER IN (
SELECT S_NUMBER
FROM EMPLOYEE
WHERE CO='ABC'
GROUP BY S_NUMBER
HAVING COUNT(*) > 1)
I've done some searching on using Group By in LINQ as well as sub-queries. I'm using LinqPad with a "C# Statement" and I came up with the following which based on some examples I found looks like it should work. However, I'm getting errors when trying to assign esn.S_NUMBER to sNumber in the anonymous object. The message says 'IGrouping' does not contain a definition for 'S_NUMBER'.
var result = from e in EMPLOYEE
where e.CO=="ABC"
group e by e.S_NUMBER into esn
select new
{
sNumber = esn.S_NUMBER
};
result.Dump();
I was under the impression that all the records would basically get put into a temp table called esn and I could be able to call the temptable.column name to assign it to my object that I will eventually return as a list.

You want to use Key instead of S_NUMBER. When grouping, the results get put into a IEnumerable<IGrouping>>. The grouping has a Key property which holds the key for that group, which in this case it's your S_NUMBER.
select new
{
sNumber = esn.Key
};
The following query should be a translation of the original SQL query. Instead of using a subquery, we're grouping and doing another from...in to "flatten" the sequence, and also checking that each grouping has a count > 1 like the original query.
var result = from e in EMPLOYEE
where e.CO=="ABC"
group e by e.S_NUMBER into esn
from e2 in esn
where esn.Count() > 1
select new
{
e.S_NUMBER,
e.A_NUMBER,
e.FIRST_NAME,
e.LAST_NAME
};

Since you're using the results of one query to filter another we can do a fairly direct transliteration of the query like so:
var result =
from e in EMPLOYEE
join f in (
from fe in EMPLOYEE
where fe.CO == 'ABC'
group null by S_NUMBER into grp
where grp.Count() > 1
select grp.Key
)
on e.S_NUMBER equals f
select new { e.S_NUMBER, e.A_NUMBER, e.FIRST_NAME, e.LAST_NAME };
Not only does this look a lot more like the original query but it should perform a bit faster (on MS SQL at least, can't speak for others) than the other form that might be simpler in LINQ but is much more complex when converted to SQL... four selects and a cross join, in my test version, vs two selects and an inner join for this one.
Of course if you prefer you can pull the inner query out as a separate IQueryable for clarity:
var filter =
from e in EMPLOYEE
where e.CO == 'ABC'
group null by S_NUMBER into grp
where grp.Count() > 1
select grp.Key;
var result =
from e in EMPLOYEE
join f in filter
on e.S_NUMBER equals f
select new { e.S_NUMBER, e.A_NUMBER, e.FIRST_NAME, e.LAST_NAME };

Append string to linq-to-sql query

I have a form with some indexes based on a document type.
I want to build my linq-to-sql query based on those index. The user might fill just some indexes or all of then.
I would need somenthing like that
Gedi.Models.OperacoesModel.indexMatrix[] IndexMatrixArr = (from a in context.sistema_Documentos
join b in context.sistema_Indexacao on a.id equals b.idDocumento
join c in context.sistema_Indexes on a.idDocType equals c.id
join d in context.sistema_DocType_Index on c.id equals d.docTypeId
where d.docTypeId == idTipo and "BUILT STRING"
orderby b.idIndice ascending
select new Gedi.Models.OperacoesModel.indexMatrix {
idDocumento = a.id,
idIndice = b.idIndice,
valor = b.valor
}).Distinct().ToArray();
This built string should be buit early in the code something like
field1 == a and field2 == b
Is this possible?

Your goal is to create expression dynamicaly, as far as I can see. And there are no way just to put string in linq query and make it work in simple linq world - that's the bad news. But I also have a good news for you - there are some way exist to create your query dynamicaly:expression tree, dynamic LINQ.

Add Join to this LINQ Query

I'm using the following query and am having trouble figuring out how to add a join into it:
var chi = Lnq.attlnks.Where(a => a.ownerid == emSysid)
.Select(c => new { sysid });
How can I join this to the "attach" table (ON attlnks.sysid = attach.sysid) and select "name" where sysid is the row id?

For joins in Linq the query expression form is typically more readable than lambda syntax - I believe this is what you are asking for:
var chi = from t in Lnq.attach
join a in Lnq.attlnks
on t.sysid equals a.sysid
where a.ownerid == emSysid
select t.name;
If there is only a single entry that should match at most, you can chain a FirstOrDefault() in this case (or other alternatives like SingleOrDefault, Single, First etc.):
var chi = (from t in Lnq.attach
join a in Lnq.attlnks
on t.sysid equals a.sysid
where a.ownerid == emSysid
select t.name).FirstOrDefault();

If I understood your problem, this should work fine:
var query = from a in attlinks
join aa in attach on a.sysid equals aa.sysid into a2
where a2.sysid == a2.ownerid
select a2.Name;