Hi I need some help coverting this sql statement to Linq, I am very new to Linq and LinqToSql, and this is my weakness, it seems like this is used frequently and I need to wrap my brain around the syntax. The Code is below.
select distinct t1.Color from [ProductAttributes] t1 join [Product] t2 on t1.Name = t2.ProductName where t1.ProductID = #productID order by t1.color
#productID is the parameter coming into the function, where I am trying to use Linq in MVC.
Thanks
It might be like this I guess
int myProductID = 1;//or whatever id you want.
MyDataContext mdc = new MyDataContext(CONNECTION_STRING_IF_NEEDED);
//MyDataContext is your datacontext generated by LinqToSql
var result = (from x in mdc.ProductAttributes
join y in Products on x.Name.equals(y.ProductName)
where x.ProductID = myProductID
orderby x.color
select x.Color).Distinct();
Note That Table names might need to be fixed.
Related
This is my SQL query
SELECT
sys.sysobjects.name Name,
sys.foreign_keys.*
FROM
sys.foreign_keys
inner join sys.sysobjects on
sys.foreign_keys.parent_object_id = sys.sysobjects.id
WHERE
referenced_object_id = OBJECT_ID(N'[dbo].[Country]')
I have installed Linqer to convert SQL to linq.
But I got an error:
SQL cannot be converted to LINQ: Table [foreign_keys] not found in the current Data Context.
I am a beginner in Linq. Can Anyone help me to convert SQL to Linq
The problem is that system views will not be picked up by Linqer. If you want to read these tables in your application, first create your own views on them, as was done here and write a query on these views.
CREATE VIEW SysObjectsView AS SELECT * FROM sys.sysobjects;
GO
CREATE VIEW SysForeignKeysView AS SELECT * FROM sys.foreign_keys;
GO
SELECT obj.name Name, fk.*
FROM SysForeignKeysView fk
INNER JOIN SysObjectsView obj ON fk.parent_object_id = obj.id
INNER JOIN SysObjectsView objfk ON fk.referenced_object_id = objfk.id
WHERE objfk.name = N'Country'
Linqer should be able to pick up these views.
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.
There are many examples of outer join using Linq to Sql, all of them hinging on DefaultIfEmpty() which is not supported with Linq to Entity Framework.
Does this mean that outer join is not possible with Linq to Entity using .NET 3.5 (I understand that DefaultIfEmpty is coming with 4.0 --- but that's not an option at this time for me)
Could somebody please provide a concise example using Linq to EntityFramework.
In LINQ to Entities, think in terms of relationships rather than SQL joins. Hence, the literal equivalent of a SQL outer join on an entity Person with a one to zero or one relationship to CustomerInfo would be:
var q = from p in Context.People
select new
{
Name = p.Name,
IsPreferredCustomer = (bool?)p.CustomerInfo.IsPreferredCustomer
};
L2E will coalesce the join, so that if CustomerInfo is null then the whole expression evaluates to null. Hence the cast to a nullable bool, because the inferred type of non-nullable bool couldn't hold that result.
For one-to-many, you generally want a hierarchy, rather than a flat, SQL-style result set:
var q = from o in Context.Orders
select new
{
OrderNo = o.OrderNo,
PartNumbers = from od in o.OrderDetails
select od.PartNumber
}
This is like a left join insofar as you still get orders with no details, but it's a graph like OO rather than a set like SQL.
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.
i'm using LINQ To SQL to perform an insert via db.table.InsertOnSubmit(). I'm wondering if there is a way to reproduce the T-SQL version of the 'where not exists (select etc etc) begin insert into etc etc end' as one single query? Thanks, Martin
LINQ has an extension method called Contains which allows for this functionality. This can be seen in the following example:
NorthwindDataContext dc = new NorthwindDataContext();
dc.Log = Console.Out;
var query =
from c in dc.Customers
where !(from o in dc.Orders
select o.CustomerID)
.Contains(c.CustomerID)
select c;
foreach (var c in query) Console.WriteLine( c );
Note the negation on the where clause!
This example was from the website here.
Nothing build in as far as I know, we would have to go about finding the row manually using where and than do the Insert.
There is a possibility of race coditions in such queries. Have a look at this thread for detailed solution :
http://social.msdn.microsoft.com/Forums/en-US/linqtosql/thread/b1a0eb5b-d5d3-41af-829f-bbbac47b7383/