How do I left outer join two tables on two fields in linq?
I have a sql:
select a.*, b.* from courselist as a
left outer join Summary as b
on a.subject = b.Subject and a.catalog =
b.Catalogno
where a.degree_id = 1
order by a.sequenceNo
Below is my linq query, but there is error underline "join", failed in the call to "Groupjoin". I don't know how to correct that.
var searchResults = (from a in db.courselist
join b in db.Summary on
new { a.subject,a.catalog } equals
new { b.Subject, b.Catalogno } into ab
where a.degree_id == 1
orderby a.degree_sequenceNo
from b in ab.DefaultIfEmpty()
select new
{
Courselist = a,
Summary = b
}
).ToList();
Thanks.
I've checked your code again,
I found it's fault
you just need to specify join parameters name like this:
new { suject = a.subject, catalog = a.catalog } equals
new { suject = b.subject, catalog = b.Catalogno } into ab
It seems you are missing the reference, the query doesn't have an error
try to use this:
using System.Linq;
The main issue when people start using LINQ is that they keep thinking in the SQL way, they design the SQL query first and then translate it to LINQ. You need to learn how to think in the LINQ way and your LINQ query will become neater and simpler. For instance, in your LINQ you don't need joins. You should use Associations/Navigation Properties instead. Check this post for more details.
There should be a relationship between courselist and Summary, in which case, you can access Summary through courselist like this:
var searchResults = (from a in db.courselist
where a.degree_id == 1
orderby a.degree_sequenceNo
select new {
Courselist = a,
Summary = a.Summary
}).ToList();
If there is no relationship between the two, then you should reconsider your design.
Related
I want to do a Linq query that joins three tables, but only returns data from two of them (the third is only joined for ordering purposes). I'm trying to order by columns that aren't in the output of the produced query, but they seem to be ignored:
var records = from q in _pdxContext.Qualifier
join aql in _pdxContext.ApplicationQualifierLink on q.Id equals aql.QualifierId
join qt in _pdxContext.QualifierType on q.QualifierTypeId equals qt.Id
where SOME_LIST.Contains(aql.ApplicationId)
orderby aql.Sequence
select new Qualifier
{
Id = q.Id,
QualifierType = new QualifierType
{
Id = qt.Id, Value = qt.Value
}
};
return records.Distinct().ToList();
The output SQL from this does NOT have an ORDER BY clause.
If I change the orderby to read like so:
orderby q.Id
... then the output SQL has the order by clause.
Does Linq ignore orderby statements when the mentioned columns aren't used in the output (as appears to be the case here)? If so, how do I order by columns not in the output?
It seems this is an SQL limitation. The error from the SQL Server engine:
"ORDER BY items must appear in the select list if SELECT DISTINCT is specified."
So, as written, I can't do what I want to do.
I ended up using:
using (var cnn = new SqlConnection(_connectionString))
{
string sql = #"select
min(q.Id) Id, q.QualifierTypeId, q.QualifierTypeId, min(q.AcaId) AcaId,
q.QualifierTypeId Id, qt.Value
from
qdb.Qualifier q
inner join qdb.QualifierType qt on qt.Id = q.QualifierTypeId
inner join ApplicationQualifierLink l on l.QualifierId = q.id
where l.ApplicationId in (" + string.Join(",", applicationIds) + #")
group by q.Text, q.QualifierTypeId, qt.Value";
qualifiers = cnn.Query<Qualifier, QualifierType, Qualifier>(sql,
(qualifier, type) =>
{
qualifier.QualifierType = type; return qualifier;
}
).ToList();
}
Note: When you attempt to use order by and distinct as in my original clause, no error is given, entity framework silently discards the order by without any error.
I've come up with the following LINQ, but I can't help thinking that I should be able to do this in one query instead of two. Can anyone help?
The idea is to inner join three tables and group by one.
var q1 = from er in ExportRules
join per in PlaceExportRules on er.ExportRulesID equals per.ExportRulesID
select new
{
PlaceID = per.PlaceID,
Description = er.Description,
Tag = er.TagName,
ExportName = per.ExportName,
ExportAddress = per.ExportAddress
};
var q2 = from p in Places
join rules in q1 on p.PlaceID equals rules.PlaceID into joined2
where joined2.Any()
orderby p.PlaceName
select new {Place = new {p.PlaceID, p.PlaceName}, joined2};
Well, you can just bracket things:
var query = from p in Places
join rules from
(from er in ExportRules
join per in PlaceExportRules
on er.ExportRulesID equals per.ExportRulesID
...)
on p.PlaceId equals rules.PlaceId into joined2
where joined2.Any()
orderby p.PlaceName
select new {Place = new {p.PlaceID, p.PlaceName}, joined2};
However, I'd personally probably leave it as two queries. It's clearer, and won't affect the performance - it's not like the first statement in your code would actually execute the query.
So we copy and paste the exact same query from LinqPad into our EF 4.3 application, pointed at the exact same database and get a different result. In LinqPad we get 2 records returned. In our application we reaise an error "Object reference not set to an instance of an object."
var Shippings = shippingRepository.All.ToArray();
var SalesOrderHeaders = salesOrderHeaderRepository.All.ToArray();
var Customers = customerRepository.All.ToArray();
var Stores = storeRepository.All.ToArray();
var Departments = departmentRepository.All.ToArray();
var toShip = from sh in Shippings
join h in SalesOrderHeaders on sh.OrderId equals h.SalesOrderHeaderId
join c in Customers on h.CustomerId equals c.CustomerId
join st in Stores on h.StoreId equals st.StoreId
join d in Departments on h.DepartmentId equals d.DepartmentId into outer
from o in outer.DefaultIfEmpty()
select new
{
OrderId = sh.OrderId,
CustomerName = c.Name,
StoreName = st.Name,
DepartmentName = (o.Name == null) ? o.Name : "None",
DeliveryDate = h.DeliveryDateTime
};
In the application code, when we remove the outer join (to add Departments) and it's associated field the query returns the same 2 records asn in LinqPad.
Does anyone have any insight into how to fix this feature?
Click on "Add a connection" in linqpad and select datacontext from assembly like
You can choose Entity Framework datacontext or Entity Framework BDContext with POCO depending upon your scenario. click next and provide path to the assembly along with connection string and you will be good to go.
In LINQPad are you actually querying against your entity model? Take a look at this link if you aren't. I had a similar problem when starting out and didn't realize I had set up a default LINQ to SQL connection earlier and was querying against that.
I wonder if anyone can help
I have a simple SQL Query which was this.
SELECT * FROM ITEM INNER JOIN CODES ON ITEM.ITEMID = CODES.ITEMID AND
ITEM.CATID = CODES.CATID
This works great, so i created the following query in LINQ
from i in Item join c in codes
on new { i.itemid, i.catid } equals new { c.itemid, c.catid }
Works great, but they i realized that i actually needed an LEFT JOIN in place of the INNER JOIN. So the sql, which now works great!, is :- (notice left join)
SELECT * FROM ITEM LEFT JOIN CODES ON ITEM.ITEMID = CODES.ITEMID AND
ITEM.CATID = CODES.CATID
So now everything seems to work and i get back the data i require using SQL i can't figure out how to do a left join in LINQ and especially how to do a left join on more than 1 field - in my case itemid and catid
I wonder if anyone can lend a hand, i am stuck :-)
I think this is the direction you need to head down:
from i in Item
join c in codes on new { i.itemid, i.catid } equals new { c.itemid, c.catid }
into grouped
from g in grouped.DefaultIfEmpty()
select new
{
ItemId = c.itemid,
CatItemId = g != null ? g.itemid : "Empty"
};
You should look: http://codingsense.wordpress.com/2009/03/08/left-join-right-join-using-linq/
The key is -->
into JoinedEmpDept
from dept in JoinedEmpDept.DefaultIfEmpty()
I'm trying to use LINQ to query the following Entity Data Model
based on this db model
I'd like to be able to pull a list of products based on ProductFacets.FacetTypeId.
Normally, I'd use joins and this wouldn't be a problem but I don't quite understand how to query many-to-many tables under the Entity DataModel.
This is an example sql query:
select p.Name, pf.FacetTypeId from Products p
inner join ProductFacets pf on p.ProductId = pf.ProductId
where pf.FacetTypeId in(8, 12)
Presuming EF 4:
var facetIds = new [] { 8, 12 };
var q = from p in Context.Products
where p.FacetTypes.Any(f => facetIds.Contains(f.FacetTypeId))
select p;
In EF (assuming the mapping is done correctly), joins are hardly ever used; navigation properties are used instead.
Your original SQL returns a tuple with repeated Name entries. With LINQ, it's often easier
to "shape" the queries into non-tuple results.
The following should be the same as the SQL, only instead of returning (Name, FacetTypeId) pairs with repeated Names, it will return a type that has a Name and a sequence of FacetTypeIds:
var facetIds = new [] { 8, 12 };
var result = from p in db.Products
select new
{
p.Name,
FacetTypeIds = from pf in p.FacetTypes
where pf.FacetTypeId == 8 || pf.FacetTypeId == 12
select pf.FacetTypeId,
};