I need compute some join operations on my data, on the result of these operations I need to use a WHERE statement.
At the moment I use this syntax below, no errors, but result is not as expected.
For sure I need Group the result for my join and apply a query on it.
Could you please provide me an example of code? Unfortunately I'm pretty new at Linq and I'm not able to do it.
var myImagesForUser = from i in context.CmsImagesContents
join c in context.CmsContents on i.ContentId equals c.ContentId
join a in context.CmsAuthors on c.AuthorId equals a.AuthorId
join u in context.aspnet_Users on a.UserId equals u.UserId
where u.UserId == (Guid)myLoggedInUser.ProviderUserKey
select i;
Probably related data for current logged in user is not present in any one or more tables (Authors, Contents, ImagesContents) and you are not getting any result due to inner join on these tables. you can try outer join instead. Look at this article for left outer join in Linq queries
Hmm. I see two things which I would do different.
1.) You forgot to call .ToList() at the end in order to execute the query. Otherwise it can be, that you misuse the LINQ Statement in the processing afterwords.
var myImagesForUser = (YOUR LINQ STATEMENT).ToList();
2.) It's strange that you need to convert the UserKey to GUID. If really needed I would do it prior the LINQ Statement and check against a local variable.
var userId = (Guid)myLoggedInUser.ProviderUserKey;
var myImagesForUser = (YOUR LINQ STATEMENT ... where u.UserId == userId ).ToList();
Perhaps this helps you to get the expected result.
where a.UserId == (Guid)myLoggedInUser.ProviderUserKey.
Related
I have this HQL where I need a subquery. I know it's not legal to make a subquery in order by, but I can't figure out how to do it
SELECT OBJECT(l) FROM InboundNotification l
INNER JOIN l.item item
WHERE l.job = ? ORDER BY (SELECT SUM(itemInst.qty)
FROM ItemInst itemInst
WHERE itemInst.receivedFromNotification_id = l.id) DESC, item.localId DESC
The above fails since I have the subquery in order by. How can I reconfigure it so this will work?
A sort in the Java code is not a option here even though it's almost as efficient.
ok, i haven't a notion of hql, but I'm gonna assume it's something like other query languages dive in here given that this question has remained unanswered for so long.
could you rewrite the query so it's something like this:
SELECT OBJECT(l), SUM(itemInst.qty) theSum
FROM InboundNotification l
INNER JOIN l.item item WHERE l.job = ?
INNER JOIN ItemInst on ItemInst.KEY = l.KEY
WHERE itemInst.receivedFromNotification_id = l.id)
GROUP BY OBJECT(l)
ORDER BY theSum
where ItemInst.KEY = l.KEY shows the appropriate relationship for your situation (if such a relationship exists)
This question already has an answer here:
In explicit LINQ-to-SQL (C#) does order matter?
(1 answer)
Closed 8 years ago.
I have this linq Query in a C# project
var query = from p in db.Posts
join a in db.Author on p.Author equals a
join u in db.Users on p.PostedBy equals u
where p.IsActive.Equals(true)
orderby p.PostedDate descending
select new ViewModel
{
...
};
If I move where p.IsActive.Equals(true) near from p in db.Posts, like
var query = from p in db.Posts
where p.IsActive.Equals(true) //Moved
join a in db.Author on p.Author equals a
join u in db.Users on p.PostedBy equals u
orderby p.PostedDate descending
select new ViewModel
{
...
};
will it make any difference to the performance of the query?
I'm not sure that it will work if you'll do it, but if you are using it against a SQL database it should not make any difference. Both queries will be translated to the SQL like this one:
SELECT ...
FROM Posts INNER JOIN Author ON ... INNER JOIN Users ON ...
WHERE Posts.IsActive = true
...
Concerning performance, I would strongly suggest to put
the joins before any where.
The reason is, that the joins already make a smaller set
(the p is selected from Posts and then only
the matching rows from Author and Users are taken.
In opposite, if you have a where near the beginning
a caresian (big) product is calculated and then filtered.
(in your special case, only one table is affected, but
the query may be altered sometime and nobody knows then
why it is slow).
Please read this SO Why is LINQ join so much faster than LINQ where
And more on your special case: as this is affecting a database,
the resulting SQL statements should be the same in both queries.
Have a look at it!
I have two classes: Property and PropertyValue. A property has several values where each value is a new revision.
When retrieving a set of properties I want to include the latest revision of the value for each property.
in T-SQL this can very efficiently be done like this:
SELECT
p.Id,
pv1.StringValue,
pv1.Revision
FROM dbo.PropertyValues pv1
LEFT JOIN dbo.PropertyValues pv2 ON pv1.Property_Id = pv2.Property_Id AND pv1.Revision < pv2.Revision
JOIN dbo.Properties p ON p.Id = pv1.Property_Id
WHERE pv2.Id IS NULL
ORDER BY p.Id
The "magic" in this query is to join on the lesser than condition and look for rows without a result forced by the LEFT JOIN.
How can I accomplish something similar using LINQ to EF?
The best thing I could come up with was:
from pv in context.PropertyValues
group pv by pv.Property into g
select g.OrderByDescending(p => p.Revision).FirstOrDefault()
It does produce the correct result but is about 10 times slower than the other.
Maybe this can help. Where db is the database context:
(
from pv1 in db.PropertyValues
from pv2 in db.PropertyValues.Where(a=>a.Property_Id==pv1.Property_Id && pv1.Revision<pv2.Revision).DefaultIfEmpty()
join p in db.Properties
on pv1.Property_Id equals p.Id
where pv2.Id==null
orderby p.Id
select new
{
p.Id,
pv1.StringValue,
pv1.Revision
}
);
Next to optimizing a query in Linq To Entities, you also have to be aware of the work it takes for the Entity Framework to translate your query to SQL and then map the results back to your objects.
Comparing a Linq To Entities query directly to a SQL query will always result in lower performance because the Entity Framework does a lot more work for you.
So it's also important to look at optimizing the steps the Entity Framework takes.
Things that could help:
Precompile your query
Pre-generate views
Decide for yourself when to open the database connection
Disable tracking (if appropriate)
Here you can find some documentation with performance strategies.
if you want to use multiple conditions (less than expression) in join you can do this like
from pv1 in db.PropertyValues
join pv2 in db.PropertyValues on new{pv1.Property_ID, Condition = pv1.Revision < pv2.Revision} equals new {pv2.Property_ID , Condition = true} into temp
from t in temp.DefaultIfEmpty()
join p in db.Properties
on pv1.Property_Id equals p.Id
where t.Id==null
orderby p.Id
select new
{
p.Id,
pv1.StringValue,
pv1.Revision
}
I want to achieve the following in Linq to Entities:
Get all Enquires that have no Application or the Application has a status != 4 (Completed)
select e.*
from Enquiry enq
left outer join Application app
on enq.enquiryid = app.enquiryid
where app.Status <> 4 or app.enquiryid is null
Has anyone done this before without using DefaultIfEmpty(), which is not supported by Linq to Entities?
I'm trying to add a filter to an IQueryable query like this:
IQueryable<Enquiry> query = Context.EnquirySet;
query = (from e in query
where e.Applications.DefaultIfEmpty()
.Where(app=>app.Status != 4).Count() >= 1
select e);
Thanks
Mark
In EF 4.0+, LEFT JOIN syntax is a little different and presents a crazy quirk:
var query = from c1 in db.Category
join c2 in db.Category on c1.CategoryID equals c2.ParentCategoryID
into ChildCategory
from cc in ChildCategory.DefaultIfEmpty()
select new CategoryObject
{
CategoryID = c1.CategoryID,
ChildName = cc.CategoryName
}
If you capture the execution of this query in SQL Server Profiler, you will see that it does indeed perform a LEFT OUTER JOIN. HOWEVER, if you have multiple LEFT JOIN ("Group Join") clauses in your Linq-to-Entity query, I have found that the self-join clause MAY actually execute as in INNER JOIN - EVEN IF THE ABOVE SYNTAX IS USED!
The resolution to that? As crazy and, according to MS, wrong as it sounds, I resolved this by changing the order of the join clauses. If the self-referencing LEFT JOIN clause was the 1st Linq Group Join, SQL Profiler reported an INNER JOIN. If the self-referencing LEFT JOIN clause was the LAST Linq Group Join, SQL Profiler reported an LEFT JOIN.
Do this:
IQueryable<Enquiry> query = Context.EnquirySet;
query = (from e in query
where (!e.Applications.Any())
|| e.Applications.Any(app => app.Status != 4)
select e);
I don't find LINQ's handling of the problem of what would be an "outer join" in SQL "goofy" at all. The key to understanding it is to think in terms of an object graph with nullable properties rather than a tabular result set.
Any() maps to EXISTS in SQL, so it's far more efficient than Count() in some cases.
Thanks guys for your help. I went for this option in the end but your solutions have helped broaden my knowledge.
IQueryable<Enquiry> query = Context.EnquirySet;
query = query.Except(from e in query
from a in e.Applications
where a.Status == 4
select e);
Because of Linq's goofy (read non-standard) way of handling outers, you have to use DefaultIfEmpty().
What you'll do is run your Linq-To-Entities query into two IEnumerables, then LEFT Join them using DefaultIfEmpty(). It may look something like:
IQueryable enq = Enquiry.Select();
IQueryable app = Application.Select();
var x = from e in enq
join a in app on e.enquiryid equals a.enquiryid
into ae
where e.Status != 4
from appEnq in ae.DefaultIfEmpty()
select e.*;
Just because you can't do it with Linq-To-Entities doesn't mean you can't do it with raw Linq.
(Note: before anyone downvotes me ... yes, I know there are more elegant ways to do this. I'm just trying to make it understandable. It's the concept that's important, right?)
Another thing to consider, if you directly reference any properties in your where clause from a left-joined group (using the into syntax) without checking for null, Entity Framework will still convert your LEFT JOIN into an INNER JOIN.
To avoid this, filter on the "from x in leftJoinedExtent" part of your query like so:
var y = from parent in thing
join child in subthing on parent.ID equals child.ParentID into childTemp
from childLJ in childTemp.Where(c => c.Visible == true).DefaultIfEmpty()
where parent.ID == 123
select new {
ParentID = parent.ID,
ChildID = childLJ.ID
};
ChildID in the anonymous type will be a nullable type and the query this generates will be a LEFT JOIN.
I have the following Linq statement:
(from order in Orders.AsEnumerable()
join component in Components.AsEnumerable()
on order.ORDER_ID equals component.ORDER_ID
join detail in Detailss.AsEnumerable()
on component.RESULT_ID equals detail.RESULT_ID
where orderRestrict.ORDER_MNEMONIC == "MyOrderText"
select new
{
Mnemonic = detail.TEST_MNEMONIC,
OrderID = component.ORDER_ID,
SeqNumber = component.SEQ_NUM
}).ToList()
I expect this to put out the following query:
select *
from Orders ord (NoLock)
join Component comp (NoLock)
on ord .ORDER_ID = comp.ORDER_ID
join Details detail (NoLock)
on comp.RESULT_TEST_NUM = detail .RESULT_TEST_NUM
where res.ORDER_MNEMONIC = 'MyOrderText'
but instead I get 3 seperate queries that select all rows from the tables. I am guessing that Linq is then filtering the values because I do get the correct values in the end.
The problem is that it takes WAY WAY too long because it is pulling down all the rows from all three tables.
Any ideas how I can fix that?
Remove the .AsEnumerable()s from the query as these are preventing the entire query being evaluated on the server.