Develop Reference

Linq to Entities join into with DefaultIfEmpty returns no results

Query below yields no results:
var query = from g in context.Groups
join sl in context.GroupDancerLinks on g.Id equals sl.GroupId into sls
join usl in context.GroupForeignDancerLinks on g.Id equals usl.GroupId into usls
from sl in sls.DefaultIfEmpty()
from usl in usls.DefaultIfEmpty()
where usl.IsLead
where sl.IsLead
select new GroupGridDTO
{
Id = g.Id,
LeadName = sl.Dancer.Name,
UnregisteredLeadName = usl.ForeignDancer.Name,
DancersCount = sls.Count(),
ForeignDancersCount = usls.Count()
};
However, when same query is modified to use FirstOrDefault, results are found:
var query = from g in context.Groups
join sl in context.GroupDancerLinks on g.Id equals sl.GroupId into sls
join usl in context.GroupForeignDancerLinks on g.Id equals usl.GroupId into usls
select new GroupGridDTO
{
Id = g.Id,
LeadName = sls.Where(sl => sl.IsLead).Select(sl => sl.Dancer.Name).FirstOrDefault(),
UnregisteredLeadName = usls.Where(usl => usl.IsLead).Select(usl => usl.ForeignDancer.Name).FirstOrDefault(),
DancersCount = sls.Count(),
ForeignDancersCount = usls.Count()
};
Looking on how queries are written, results should be the same. What is wrong with first query?
EDIT:
Tables "Groups" and "ForeignDancers" are related with M2M table "GroupForeignDancerLinks".
Tables "Groups" and "Dancers" are related with M2M table "GroupDancerLinks".

In the first query if a given row has IsLead as being false the row will be removed entirely. In the second query the row exists, you just leave the corresponding item's name blank.
The queries are also semantically different if there are multiple matches for a given row in the left table. The first will list all matching rows, the latter only the first.

join not returning all elements in left table C# lambda

I have 2 tables, the left table has data like this:
I do a left join with another table with the following expression:
var result = posicion.Join(fact,
p => p.Cod_articulo,
f => f.Cod_articulo,
(p, f) => new { p.Posicion, p.Cant_historico,
p.Cod_articulo, f.Cantidad_facturada });
The problem is that the results don't include some items from the left table as seen below:
As you can see in the result there is no data for position 3, 6 etc. what would be missing from my join?

You need to do group join (that is left join in Linq). It's better to do with query syntax:
from p in posicion
join f in fact
on p.Cod_articulo equals f.Cod_articulo into g // GroupJoin
from pf in g.DefaultIfEmpty()
select new {
p.Posicion,
p.Cant_historico,
p.Cod_articulo,
Cantidad_facturada = (pf == null) ? null : pf.Cantidad_facturada
}
This query selects all facts corresponding to posicion p into group g. Then from each group we select results, even if there is no corresponding facts for current posicion (that is DefaultIfEmpty case).
Lambda syntax will be much less readable:
posicion.GroupJoin(fact,
p => p.Cod_articulo,
f => f.Cod_articulo,
(p, g) => new { p, g })
.SelectMany(x => x.g.DefaultIfEmpty(), (x, pf) => new {
x.p.Posicion,
x.p.Cant_historico,
x.p.Cod_articulo,
Cantidad_facturada = (pf == null) ? null : pf.Cantidad_facturada
});
Consider also reading this MSDN article: How to: Perform Left Outer Joins

what would be missing from my join?
Presumably there are no entries in fact which have the corresponding Cod_articulo values (e.g. 60155 for posicion 3). Join in LINQ represents an inner join, where there has to be an entry in both sources in order to create an appropriate result.
If you want a left join, you'd typically use GroupJoin, so that each element on the "left" side ends up matching a group of entries from the "right" side, where that group may be empty.

Converting SQL to LINQ query when I cannot use "IN"

I'm trying to convert this very simple piece of SQL to LINQ:
select * from Projects p
inner join Documents d
on p.ProjectID = d.ProjectID
left join Revisions r
on r.DocumentID = d.DocumentID
and r.RevisionID IN (SELECT max(r2.RevisionID) FROM Revisions r2 GROUP BY r2.DocumentID)
WHERE p.ProjectID = 21 -- Query string in code
This says, if any revisions exist for a document, return me the highest revision ID. As it's a left join, if not revisions exist, I still want the results returned.
This works as expected, any revisions which exist are shown (and the highest revision ID is returned) and so are all documents without any revisions.
When trying to write this using LINQ, I only get results where revisions exist for a document.
Here is my attempt so far:
var query = from p in db.Projects
join d in db.Documents on new { ProjectID = p.ProjectID } equals new { ProjectID = Convert.ToInt32(d.ProjectID) }
join r in db.Revisions on new { DocumentID = d.DocumentID } equals new { DocumentID = Convert.ToInt32(r.DocumentID) } into r_join
from r in r_join.DefaultIfEmpty()
where
(from r2 in db.Revisions
group r2 by new { r2.DocumentID }
into g
select new { MaxRevisionID = g.Max(x => x.RevisionID) }).Contains(
new { MaxRevisionID = r.RevisionID }) &&
p.ProjectID == Convert.ToInt32(Request.QueryString["projectId"])
select new { d.DocumentID, d.DocumentNumber, d.DocumentTitle, RevisionNumber = r.RevisionNumber ?? "<No rev>", Status = r.DocumentStatuse == null ? "<Not set>" : r.DocumentStatuse.Status };
I'm not very good at LINQ and have been using the converter "Linqer" to help me out, but when trying I get the following message:
"SQL cannot be converted to LINQ: Only "=" operator in JOIN expression
can be used. "IN" operator cannot be converted."
You'll see I have .DefaultIfEmpty() on the revisions table. If I remove the where ( piece of code which does the grouping, I get the desired results whether or not a revision exists for a document or not. But the where clause should return the highest revision number for a document IF there is a link, if not I still want to return all the other data. Unlike my SQL code, this doesn't happen. It only ever returns me data where there is a link to the revisions table.
I hope that makes a little bit of sense. The group by code is what is messing up my result set. Regardless if there is a link to the revisions table, I still want my results returned. Please help!
Thanks.
=======
The code I am now using thanks to Gert.
var query = from p in db.Projects
from d in p.Documents
where p.ProjectID == Convert.ToInt32(Request.QueryString["projectId"])
select new
{
p.ProjectID,
d.DocumentNumber,
d.DocumentID,
d.DocumentTitle,
Status = d.Revisions
.OrderByDescending(rn => rn.RevisionID)
.FirstOrDefault().DocumentStatuse.Status,
RevisionNumber = d.Revisions
.OrderByDescending(rn => rn.RevisionID)
.FirstOrDefault().RevisionNumber
};
gvDocumentSelection.DataSource = query;
gvDocumentSelection.DataBind();
Although this works, you'll see I'm selecting two fields from the revisions table by running the same code, but selecting two different fields. I'm guessing there is a better, more efficient way to do this? Ideally I would like to join on the revisions table in case I need to access more fields, but then I'm left with the same grouping problem again.
Status = d.Revisions
.OrderByDescending(rn => rn.RevisionID)
.FirstOrDefault().DocumentStatuse.Status,
RevisionNumber = d.Revisions
.OrderByDescending(rn => rn.RevisionID)
.FirstOrDefault().RevisionNumber
Final working code:
var query = from p in db.Projects
from d in p.Documents
where p.ProjectID == Convert.ToInt32(Request.QueryString["projectId"])
select new
{
p.ProjectID,
d.DocumentNumber,
d.DocumentID,
d.DocumentTitle,
LastRevision = d.Revisions
.OrderByDescending(rn => rn.RevisionID)
.FirstOrDefault()
};
var results = from x in query
select
new
{
x.ProjectID,
x.DocumentNumber,
x.DocumentID,
x.DocumentTitle,
x.LastRevision.RevisionNumber,
x.LastRevision.DocumentStatuse.Status
};
gvDocumentSelection.DataSource = results;
gvDocumentSelection.DataBind();

If you've got 1:n navigation properties there is a much simpler (and recommended) way to achieve this:
from p in db.Projects
from d in p.Documents
select new { p, d,
LastRevision = d.Revisions
.OrderByDescending(r => r.RevisionId)
.FirstOrDefault() }
Without navigation properties it is similar:
from p in db.Projects
join d in db.Documents on new { ProjectID = p.ProjectID }
equals new { ProjectID = Convert.ToInt32(d.ProjectID) }
select new { p, d,
LastRevision = db.Revisions
.Where(r => d.DocumentID = Convert.ToInt32(r.DocumentID))
.OrderByDescending(r => r.RevisionId)
.FirstOrDefault() }
Edit
You can amend this very wide base query with all kinds of projections, like:
from x in query select new { x.p.ProjectName,
x.d.DocumentName,
x.LastRevision.DocumentStatus.Status,
x.LastRevision.FieldA,
x.LastRevision.FieldB
}

Conditional Join In LINQ?

I am trying to write a query that grabs information from one database and joins it to information in a different database.
TableA
idA
valueA
idB
TableB
idB
valueB
The tricky part is that in TableA, idB isn't always defined, so when I do a normal join, I only get results where TableA has a idB value. What I want is to be able to grab all of the information from TableA even if it doesn't have a corresponding idB value.

Here is a query expression syntax version of the left join to follow up on tvanfosson's answer.
var query = from rowA in db.TableA
join rowB in db.TableB
on rowA.idB equals rowB.idB into b
from item in b.DefaultIfEmpty()
select new
{
idA = rowA.idA,
valueA = rowA.valueA,
idB = rowA.idB,
valueB = item != null ? item.valueB : 0 // or other default value
};

Use a left outer join by checking if the value returned from the right hand side is null and supplying a default value for that case.
var q = db.TableA.Join( db.TableA,
a => a.idB,
b => b.idB,
(a,b) => new
{
A = a.ValueA,
B = b == null ? null : b.ValueB
});

You can do a left outer join in LINQ with SelectMany (directly calling Queryable methods) or in comprehension syntax join ... into:
var results = from a in db.TableA
join b in db.TableB on a.idB equals b.idB
into found
select new {
A = a,
Bs = found
};
In the output Bs will be IEnumerable<typeof-db-TableB>

Left Join Example:
var leftOuterJoinQuery =
from category in categories
join prod in products on category.ID equals prod.CategoryID into prodGroup
from item in prodGroup.DefaultIfEmpty(new Product{Name = String.Empty, CategoryID = 0})
select new { CatName = category.Name, ProdName = item.Name };

What is the syntax for an inner join in LINQ to SQL?

I'm writing a LINQ to SQL statement, and I'm after the standard syntax for a normal inner join with an ON clause in C#.
How do you represent the following in LINQ to SQL:
select DealerContact.*
from Dealer
inner join DealerContact on Dealer.DealerID = DealerContact.DealerID

It goes something like:
from t1 in db.Table1
join t2 in db.Table2 on t1.field equals t2.field
select new { t1.field2, t2.field3}
It would be nice to have sensible names and fields for your tables for a better example. :)
Update
I think for your query this might be more appropriate:
var dealercontacts = from contact in DealerContact
join dealer in Dealer on contact.DealerId equals dealer.ID
select contact;
Since you are looking for the contacts, not the dealers.

And because I prefer the expression chain syntax, here is how you do it with that:
var dealerContracts = DealerContact.Join(Dealer,
contact => contact.DealerId,
dealer => dealer.DealerId,
(contact, dealer) => contact);

To extend the expression chain syntax answer by Clever Human:
If you wanted to do things (like filter or select) on fields from both tables being joined together -- instead on just one of those two tables -- you could create a new object in the lambda expression of the final parameter to the Join method incorporating both of those tables, for example:
var dealerInfo = DealerContact.Join(Dealer,
dc => dc.DealerId,
d => d.DealerId,
(dc, d) => new { DealerContact = dc, Dealer = d })
.Where(dc_d => dc_d.Dealer.FirstName == "Glenn"
&& dc_d.DealerContact.City == "Chicago")
.Select(dc_d => new {
dc_d.Dealer.DealerID,
dc_d.Dealer.FirstName,
dc_d.Dealer.LastName,
dc_d.DealerContact.City,
dc_d.DealerContact.State });
The interesting part is the lambda expression in line 4 of that example:
(dc, d) => new { DealerContact = dc, Dealer = d }
...where we construct a new anonymous-type object which has as properties the DealerContact and Dealer records, along with all of their fields.
We can then use fields from those records as we filter and select the results, as demonstrated by the remainder of the example, which uses dc_d as a name for the anonymous object we built which has both the DealerContact and Dealer records as its properties.

var results = from c in db.Companies
join cn in db.Countries on c.CountryID equals cn.ID
join ct in db.Cities on c.CityID equals ct.ID
join sect in db.Sectors on c.SectorID equals sect.ID
where (c.CountryID == cn.ID) && (c.CityID == ct.ID) && (c.SectorID == company.SectorID) && (company.SectorID == sect.ID)
select new { country = cn.Name, city = ct.Name, c.ID, c.Name, c.Address1, c.Address2, c.Address3, c.CountryID, c.CityID, c.Region, c.PostCode, c.Telephone, c.Website, c.SectorID, Status = (ContactStatus)c.StatusID, sector = sect.Name };
return results.ToList();

You create a foreign key, and LINQ-to-SQL creates navigation properties for you. Each Dealer will then have a collection of DealerContacts which you can select, filter, and manipulate.
from contact in dealer.DealerContacts select contact
or
context.Dealers.Select(d => d.DealerContacts)
If you're not using navigation properties, you're missing out one of the main benefits on LINQ-to-SQL - the part that maps the object graph.

Use Linq Join operator:
var q = from d in Dealer
join dc in DealerConact on d.DealerID equals dc.DealerID
select dc;

basically LINQ join operator provides no benefit for SQL. I.e. the following query
var r = from dealer in db.Dealers
from contact in db.DealerContact
where dealer.DealerID == contact.DealerID
select dealerContact;
will result in INNER JOIN in SQL
join is useful for IEnumerable<> because it is more efficient:
from contact in db.DealerContact
clause would be re-executed for every dealer
But for IQueryable<> it is not the case. Also join is less flexible.

Actually, often it is better not to join, in linq that is. When there are navigation properties a very succinct way to write your linq statement is:
from dealer in db.Dealers
from contact in dealer.DealerContacts
select new { whatever you need from dealer or contact }
It translates to a where clause:
SELECT <columns>
FROM Dealer, DealerContact
WHERE Dealer.DealerID = DealerContact.DealerID

Inner join two tables in linq C#
var result = from q1 in table1
join q2 in table2
on q1.Customer_Id equals q2.Customer_Id
select new { q1.Name, q1.Mobile, q2.Purchase, q2.Dates }

Use LINQ joins to perform Inner Join.
var employeeInfo = from emp in db.Employees
join dept in db.Departments
on emp.Eid equals dept.Eid
select new
{
emp.Ename,
dept.Dname,
emp.Elocation
};

Try this :
var data =(from t1 in dataContext.Table1 join
t2 in dataContext.Table2 on
t1.field equals t2.field
orderby t1.Id select t1).ToList();

OperationDataContext odDataContext = new OperationDataContext();
var studentInfo = from student in odDataContext.STUDENTs
join course in odDataContext.COURSEs
on student.course_id equals course.course_id
select new { student.student_name, student.student_city, course.course_name, course.course_desc };
Where student and course tables have primary key and foreign key relationship

try instead this,
var dealer = from d in Dealer
join dc in DealerContact on d.DealerID equals dc.DealerID
select d;

var Data= (from dealer in Dealer join dealercontact in DealerContact on dealer.ID equals dealercontact.DealerID
select new{
dealer.Id,
dealercontact.ContactName
}).ToList();

var data=(from t in db.your tableName(t1)
join s in db.yourothertablename(t2) on t1.fieldname equals t2.feldname
(where condtion)).tolist();

var list = (from u in db.Users join c in db.Customers on u.CustomerId equals c.CustomerId where u.Username == username
select new {u.UserId, u.CustomerId, u.ClientId, u.RoleId, u.Username, u.Email, u.Password, u.Salt, u.Hint1, u.Hint2, u.Hint3, u.Locked, u.Active,c.ProfilePic}).First();
Write table names you want, and initialize the select to get the result of fields.

from d1 in DealerContrac join d2 in DealerContrac on d1.dealearid equals d2.dealerid select new {dealercontract.*}

One Best example
Table Names : TBL_Emp and TBL_Dep
var result = from emp in TBL_Emp join dep in TBL_Dep on emp.id=dep.id
select new
{
emp.Name;
emp.Address
dep.Department_Name
}
foreach(char item in result)
{ // to do}