I have the following linq statement. It is returning everything from allStores. I removed the DefaultIfEmpty and then it only returned the stores that are in both the subs table AND the allStores table. I need it to return all stores that are in subs table even if they are not in the allStores table. I have tried a few different things and have moved around the defaultifempty but can't seem to get it to return what I need.
I need stores 1,2,4,5 returned (everything in table 2) and I need to pull the division ID for the storeId from table 3 and the address from table 3. Even if the store is not in the address table or not in divisionID table I still need the storeID returned in the query.
from a in allStores
join sub in subs on a.DivisionId equals sub.DivisionId
join d in divs on new { a.DivisionId, a.StoreId } equals new { d.DivisionId, d.StoreId } into s
from selected in s.DefaultIfEmpty()
StoreId
Address
1
1234 Elm St.
2
5678 Maple St.
3
9101 Bella Ave.
4
1234 Meadow Dr.
StoreId
StoreStatus
1
Closed
2
Open
4
Open
5
Open
StoreId
DivisionId
1
12
2
14
3
16
4
18
5
20
You're looking for multiple left joins. I've noticed that in this area of questions there are a lot of near duplicates. But many of these only discuss one left join, others don't describe a need to deal with null reference exceptions, and still others describe linq-to-sql or similar and not linq-to-objects, which seems to be your case here.
Your sample data is not complete, so I'm just going to trust that you are joining on the right fields. You're close with your join on divs. But you need to do the same thing for subs, and you need to add a select segment that outputs the final shape of each row extracting properties from each source.
var results =
from a in allStores
// left join allScores with "subs"
join sub in subs on a.DivisionId equals sub.DivisionId into subG
from sub in subG.DefaultIfEmpty()
// left join allScores with divs
join div in divs
on new { a.DivisonId, a.StoreId }
equals new { div.DivisionId, div.StoreId }
into divG
from div in divG.DefaultIfEmpty()
// select properties from all sources, keeping in mind some are null
select new {
a.StoreId,
a.StoreStatus,
sub?.Address, // if you have a newer version of c#
DivisonId = div == null ? null : (int?)div.DivisionId // if you have an older version of c#
};
Related
I am trying to get a list from the database containing two or more lists inside that list.(using .net core, entity framework).Assume I have two table call header and details table.
Header Table
Detail Table
And I want the result like this:
{
"data":[
{
"Country":"Singapore",
"Hospital_List":[
{
"Hospital_Name":"SG Host A"
},
{
"Hospital_Name":"SG Host A"
}
]
},
{
}
]
}
I only know two ways to get the result like this,First Way, select Country list data with blank Hospital list as List,then for loop that list to select related Hospital list from db again.
And Second Way,select Country list data with blank Hospital list as IQueryable List,and then select related Hospital list via jointing with Hospital Table.So my question is
Which way should i used to get higher performance? And Is any other way?
Please remember there has a lot of field and data in my real table.
For loop give give you the lowest perfomance, because you will create SQL query for each iteration. Instead of this, try following solution:
from hospital in hospitals
group hospital by hospital.CID into gh
join country in countries
on gh.FirstOrDefault().CID equals country.CID
select new
{
Country = country.Country,
Hospital_List = from h in gh select h
}
EDITED:
And if your model created right you can use this code:
from hospital in hospitals
join country in countries
on hospital.Country equals country
group hospital by hospital.CID into gh
select new
{
Country = from h in gh select h.Country.Country,
Hospital_List = from h in gh select h
}
When I use the following Linq query in LinqPad I get 25 results returned:
var result = (from l in LandlordPreferences
where l.Name == "Wants Student" && l.IsSelected == true
join t in Tenants on l.IsSelected equals t.IsStudent
select new { Tenant = t});
result.Dump();
When I add .Distinct() to the end I only get 5 results returned, so, I'm guessing I'm getting 5 instances of each result when the above is used.
I'm new to Linq, so I'm wondering if this is because of a poorly built query? Or is this the way Linq always behaves? Surely not - if I returned 500 rows with .Distinct(), does that mean without it there's 2,500 returned? Would this compromise performance?
It's a poorly built query.
You are joining LandlordPreferences with Tenants on a boolean value instead of a foreign key.
So, most likely, you have 5 selected land lords and 5 tenants that are students. Each student will be returned for each land lord: 5 x 5 = 25. This is a cartesian product and has nothing to do with LINQ. A similar query in SQL would behave the same.
If you would add the land lord to your result (select new { Tenant = t, Landlord = l }), you would see that no two results are actually the same.
If you can't fix the query somehow, Distinct is your only option.
I have a database with customers orders.
I want to use Linq (to EF) to query the db to bring back the last(most recent) 3,4...n orders for every customer.
Note:
Customer 1 may have just made 12 orders in the last hr; but customer 2 may not have made any since last week.
I cant for the life of me work out how to write query in linq (lambda expressions) to get the data set back.
Any good ideas?
Edit:
Customers and orders is a simplification. The table I am querying is actually a record of outbound messages to various web services. It just seemed easer to describe as customers and orders. The relationship is the same.
I am building a task that checks the last n messages for each web service to see if there were any failures. We are wanting a semi real time Health status of the webservices.
#CoreySunwold
My table Looks a bit like this:
MessageID, WebserviceID, SentTime, Status, Message, Error,
Or from a customer/order context if it makes it easer:
OrderID, CustomerID, StatusChangedDate, Status, WidgetName, Comments
Edit 2:
I eventually worked out something
(Hat tip to #StephenChung who basically came up with the exact same, but in classic linq)
var q = myTable.Where(d => d.EndTime > DateTime.Now.AddDays(-1))
.GroupBy(g => g.ConfigID)
.Select(g =>new
{
ConfigID = g.Key,
Data = g.OrderByDescending(d => d.EndTime)
.Take(3).Select(s => new
{
s.Status,
s.SentTime
})
}).ToList();
It does take a while to execute. So I am not sure if this is the most efficient expression.
This should give the last 3 orders of each customer (if having orders at all):
from o in db.Orders
group o by o.CustomerID into g
select new {
CustomerID=g.Key,
LastOrders=g.OrderByDescending(o => o.TimeEntered).Take(3).ToList()
}
However, I suspect this will force the database to return the entire Orders table before picking out the last 3 for each customer. Check the SQL generated.
If you need to optimize, you'll have to manually construct a SQL to only return up to the last 3, then make it into a view.
You can use SelectMany for this purpose:
customers.SelectMany(x=>x.orders.OrderByDescending(y=>y.Date).Take(n)).ToList();
How about this? I know it'll work with regular collections but don't know about EF.
yourCollection.OrderByDescending(item=>item.Date).Take(n);
var ordersByCustomer =
db.Customers.Select(c=>c.Orders.OrderByDescending(o=>o.OrderID).Take(n));
This will return the orders grouped by customer.
var orders = orders.Where(x => x.CustomerID == 1).OrderByDescending(x=>x.Date).Take(4);
This will take last 4 orders. Specific query depends on your table / entity structure.
Btw: You can take x as a order. So you can read it like: Get orders where order.CustomerID is equal to 1, OrderThem by order.Date and take first 4 'rows'.
Somebody might correct me here, but i think doing this is linq with a single query is probably very difficult if not impossible. I would use a store procedure and something like this
select
*
,RANK() OVER (PARTITION BY c.id ORDER BY o.order_time DESC) AS 'RANK'
from
customers c
inner join
order o
on
o.cust_id = c.id
where
RANK < 10 -- this is "n"
I've not used this syntax for a while so it might not be quite right, but if i understand the question then i think this is the best approach.
I have a database table that holds parent and child records much like a Categories table. The ParentID field of this table holds the ID of that record's parent record...
My table columns are: SectionID, Title, Number, ParentID, Active
I only plan to allow my parent to child relationship go two levels deep. So I have a section and a sub section and that it.
I need to output this data into my MVC view page in an outline fashion like so...
Section 1
Sub-Section 1 of 1
Sub-Section 2 of 1
Sub-Section 3 of 1
Section 2
Sub-Section 1 of 2
Sub-Section 2 of 2
Sub-Section 3 of 2
Section 3
I am using Entity Framework 4.0 and MVC 2.0 and have never tried something like this with LINQ. I have a FK set up on the section table mapping the ParentID back to the SectionID hoping EF would create a complex "Section" type with the Sub-Sections as a property of type list of Sections but maybe I did not set things up correctly.
So I am guessing I can still get the end result using a LINQ query. Can someone point me to some sample code that could provide a solution or possibly a hint in the right direction?
Update:
I was able to straighten out my EDMX so that I can get the sub-sections for each section as a property of type list, but now I realize I need to sort the related entities.
var sections = from section in dataContext.Sections
where section.Active == true && section.ParentID == 0
orderby section.Number
select new Section
{
SectionID = section.SectionID,
Title = section.Title,
Number = section.Number,
ParentID = section.ParentID,
Timestamp = section.Timestamp,
Active = section.Active,
Children = section.Children.OrderBy(c => c.Number)
};
produces the following error.
Cannot implicitly convert type 'System.Linq.IOrderedEnumerable' to 'System.Data.Objects.DataClasses.EntityCollection'
Your model has two navigation properties Sections1 and Section1. Rename the first one to Children and the second one to Parent.
Depending on whether you have a root Section or perhaps have each top-level section parented to itself (or instead make parent nullable?), your query might look something like:-
// assume top sections are ones where parent == self
var topSections = context.Sections.Where(section => section.ParentId == SectionId);
// now put them in order (might have multiple orderings depending on input, pick one)
topSections = topSections.OrderBy(section => section.Title);
// now get the children in order using an anonymous type for the projection
var result = topSections.Select(section => new {top = section, children = section.Children.OrderBy(child => child.Title)});
For some linq examples:
http://msdn.microsoft.com/en-us/vcsharp/aa336746.aspx
This covers pretty much all of the linq operations, have a look in particular at GroupBy. The key is to understand the input and output of each piece in order to orchestrate several in series and there is no shortcut but to learn what they do so you know what's at hand. Linq expressions are just combinations of these operations with some syntactic sugar.
I have 3 tables: A, B and C.
Table A is in relation (n:1) with B and with C.
Typically I store in A the B.Id (or the C.Id) and the table name.
e.g.
A.ParentId = 1
A.TableName = "B"
A.ParentId = 1
A.TableName = "C"
A.ParentId = 2
A.TableName = "B"
Is it a good solution? Are there any other solutions?
Why not 2 parentid columns?
A.ParentIdB = 1
A.ParentIdC = 3
Another possibility is to introduce another table Content (D) that serves as a "supertype" to Posts and Images. Then a row in Comments (A) would reference a primary key in Content as would each row in Posts (B) and Images (D). Any common fields in Posts and Images would be moved to Content (perhaps "title" or "date") and those original tables would then only contain information specific to a post or image (perhaps "body" or "resolution"). This would make it easier to perform joins than having the table names in a field, but it does mean that a real-world entity could be both a post and a comment (or indeed, be multiply a post or comment!). Really, though, it depends on the situation that you're trying to model.