I have a Linq query that returns three data elements.
var billingDateResults = from s in Subscriptions
.Where(s => (s.ProductCode.Contains("myCode")))
select { g.ID, BillingDate =s.BILL_THRU, s.ProductCode};
I would like to do distinct type of query on this to limit the results to one record per ID.
var billingDateResults = from s in Subscriptions
.Where(s => (s.ProductCode.Contains("myCode")))
group s by s.ID into g
select g.FirstOrDefault();
This works but now returns all of the fields in the records and I would like to minimize the amount of data by limiting the results to only the 3 fields in the first example.
What is a good way to do this?
TIA
Group by those three fields then.
var billingDateResults =
from s in Subscriptions
where s.ProductCode.Contains("myCode")
group new
{
g.ID,
BillingDate = s.BILL_THRU,
s.ProductCode
} by s.ID into g
select g.First(); // FirstOrDefault is not necessary, the groups will be non-empty
Related
I'm working on a LINQ query that joins three tables. For the Orders and OrderInfo table I expect a single record in each table for a given order id. However for the ShipRate table, there could be 0, 1 or more records for a given order id. So for this table I am using a left outer join. The query shown below is working if 0 or 1 records exist in the ShipRate table, but for instances where the number of records is > 1, I need to select only the most recent ShipRate record. I tried to do this by replacing the line:
from shipRate in sr.DefaultIfEmpty()
with this:
from shipRate in sr.OrderByDescending(r => r.CreateDate).Take(1).DefaultIfEmpty()
but the query takes forever, as if it is loading the entire ShipRate table. Where have I gone wrong?
var query = (from order in db.Orders
join info in db.OrderInfo
on order.OrderId equals info.OrderId
join shipRate in db.ShipRate
on info.OrderId equals shipRate.OrderId
into sr
from shipRate in sr.DefaultIfEmpty()
where order.OrderId == orderId
select new
{
OrderId = order.OrderId,
OrderDetail = info.OrderDetail,
Carrier = shipRate.Carrier
}).SingleOrDefault();
With a proper model definition your query would be like:
var query = (from order in db.Orders
where order.OrderId == orderId
select new
{
OrderId = order.OrderId,
OrderDetail = order.OrderInfo.OrderDetail,
Carrier = order.OrderInfo.ShipRates.OrderBy(sr =>sr.CreateDate).FirstOrDefault()
}).SingleOrDefault();
I can't be sure though, because you didn't supply sample data and model.
Cetin Basoz's answer is a good one: ideally you'd set up your model in a way that allows you to use navigation properties. If you're using a model generated from your database schema, that typically means setting up foreign and primary keys properly.
If you can't do that, you should still be able to get a similar effect by writing SQL like this:
var query = (from order in db.Orders
where order.OrderId == orderId
let orderInfo = db.OrderInfo.FirstOrDefault(info => order.OrderId == info.OrderId)
let currentShipRate = db.ShipRate
.Where(shipRate => info.OrderId == shipRate.OrderId)
.OrderByDescending(shipRate => shipRate.CreateDate)
.FirstOrDefault()
select new
{
OrderId = order.OrderId,
OrderDetail = orderInfo.OrderDetail,
Carrier = currentShipRate.Carrier
}).SingleOrDefault();
However, LINQ to SQL isn't nearly as good at building advanced queries as Entity Framework, and the symptoms you're describing might be an indication that it's actually doing multiple database round-trips instead of a join. I'd recommend logging the query that you're producing (prior to the .SingleOrDefault()) either by calling .ToString() on the query or by executing your query in LINQPad and clicking on the SQL tab. That might give you a clue as to why the query is misbehaving.
There seems to be a one-to-one relation between Orders and OrderInfos: every Order has exactly one OrderInfo, and every OrderInfo is the info of exactly one Order, namely the Order that the foreign key OrderId refers to.
On the other hand, there seems to be a one-to-many relation between Orders and ShipRates. Every Order has zero or more ShipRates, every ShipRate is a ShipRate of exactly one Order, namely the Order that the foreign key OrderId refers to.
You want several properties of "Orders, each Order with its one and only OrderInfo and its zero or more ShipRates"
Whenever you have a one-to-many relation, and you want "items with their zero or more sub-items", like Schools with their Students, Customers with their Orders, or in your case: Orders with their ShipRates, consider to use one of the overloads of Queryable.GroupJoin
In the other direction: if you want an item with its one and only other item that the foreign key refers to, like Student with the School he attends, Order with the Customer who created the Order, or Order with its one and only OrderInfo, use Queryable.Join.
I mostly use the overload of GroupJoin that has a parameter resultSelector, so I can select exactly what properties I want.
int orderId = ...
var ordersWithShipRates = dbContext.Orders.GroupJoin(dbContext.ShipRates,
order => order.Id, // from every Order take the primary key
shipRate => shipRate.OrderId, // from every ShipRate take the foreign key to Order
// parameter resultSelector: from every Order, with its zero or more ShipRates
// make one new
(order, shipRatesOfThisOrder) => new
{
// Select the Order properties that you plan to use:
Id = order.Id,
Date = order.Date,
...
ShipRates = shipRatesOfThisOrder.Select(shipRate => new
{
// Select the ShipRate properties that you plan to use:
Id = shipRate.Id,
Value = shipRate.Value,
...
})
.ToList(),
// A simple join to get the one and only OrderInfo
OrderInfo = dbContext.OrderInfos.Where(orderInfo => orderInfo.Id == order.Id)
.Select(orderInfo => new
{
// Select the orderInfo properties that you plan to use
Name = orderInfo.Name,
...
})
.FirstOrDefault(),
});
I'm having trouble writing my LINQ query.
Here's the scenario:
I have 2 databases: A and B
In database A: I have a tableX which has the following fields: Employee ID, Name, Address, Phone, ..., Active
In database B: I have a tableY which has the following fields: Employee ID, Visible, Order
the number of records in table Y is less than or equal to the number of records in table X.
Basically, I need to extract the employee records from table X who have the attribute 'Visible' (in table Y) set to True and would like to sort them using the 'Order' attribute.
This is what I have so far:
ADataContext dbA = new ADataContext();
BDataContext dbB = new BDataContext();
//Get the list of records from tableY where 'Visbile' is set to True
var List = dbB.tableY
.Where(x => x.Visible == true).OrderBy(x => x.Order)
.ToList();
//Extract the list of employee IDs
IEnumerable<int> ids = List.Select(x => x.EmployeeID).Distinct();
var employees = dbA.tableX
.Where(x => ids.Contains(x.EmployeeID) && x.Active == true)
.ToList();
I'm able to get the correct list of employees, but cannot figure out how to apply the sorting order (present in tableY) on tableX
Currently, regardless of the order specified in tableY, the records returned from tableX are sorted as they were entered in the table (oldest to most recent).
Any ideas how I can fix my query.
Thanks,
I've re-written it all as a single query:
var employees =
from x in dbA.tableX
where x.Active
from y in dbB.tableY
where x.EmployeeID == y.EmployeeID
orderby y.Order
select x;
Currently learning Linq to Entity. I been successful, but came stumped with the orderby clause and its use with multiple tables.
var query = from k in contxt.pages.Include("keywords")
where k.ID == vals.pageId select k;
My understanding with the code above is it creates an inner join where ID is equal to pageId.
So what I am having a difficult time visualizing is how I would perform an orderby on both tables?
I would like to sort on both tables.
I have tried:
var query = from k in contxt.pages.Include("keywords") where k.ID == vals.pageId orderby k.keywords.**?** select k;
The question mark is not supposed to be there. I am showing that the column that I would like to sort by isn't there. Trying this k.Kegwords. doesn't show the column.
I would write a SQL query as follows:
string query = "SELECT pages.page, pages.title, pages.descp, keywords.keyword
FROM pages INNER JOIN keywords ON pages.ID = keywords.pageID
ORDER BY keywords.sort, pages.page";
pages and keywords have a 1 to many relationship, which FK keywords.
Thank you,
deDogs
Here you go.
var result = (from x in pages
join y in keywords on x.ID equals y.pageID
orderby y.sort, x.page
select new
{
x.Page,
x.title,
x.descp,
y.keyword
});
I have two classes, basically one holds Members and the other Sessions.
They are joined together with a common field called "name". There is one member but can be many Sessions.
So if I do a standard join I get back 1 member and many sessions. I just want to get back the first row of sessions.
The session has field called SessioEndTime. So I need to order by DESC on this to pick out the first record.
This is my linq; I have returns too many. I think I need a subquery but I am a little confused.
var sessions = from m in this.members
join s in this.sessions
on m.Name equals s.Name
select new { MemberName = m.Name, SessionTime = s.SessioEndTime};
Edit
To make it clear, imagine I have five members, each member has NUMEROUS sessions. I just wish to receive my five members but with only one session each, that session is the LATEST session which can be got from the SessioEndTime.
Try this:
var sessions =
from m in this.members
join s in
(
from se in this.sessions
group se by se.Name into g
select new {Name = g.Key, SessioEndTime = g.Max(a=>a.SessioEndTime)}
)
on m.Name equals s.Name
select new { MemberName = m.Name, SessionTime = s.SessioEndTime}
Just wanted to check if there is way to do distinct by multiple columns. Thanks in advance!!!
BTW, I found a great LINQ extension here but need some guidance to use it for multiple columns
Well, you can do the projection first:
var qry = db.Customers.Select(cust => new {cust.ID, cust.Name, cust.Region})
.Distinct();
Or in query syntax:
var qry = (from cust in db.Customers
select new {cust.ID, cust.Name, cust.Region}).Distinct();
That do?
Instead of Distinct you can use Groupby and then selecting the Top Most record of each group
How to LINQ Distinct by Multiple Fields without anonymous types
return from o in objEntity
group o by new
{
o.Field1,
o.Field2,
o.Field3,
o.Field4,
o.Field5
} into grp
select grp.FirstOrDefault();
This will give you the EntityObject Rather than the AnonymousType
By "distinct by multiple columns" what you really mean is a group by.
When you ask for distinct, it means that you are getting ALL the distinct rows, or, a group by using all the columns in the table.
If you want to only get distinct groupings for a subset of the columns, then use a group by in your clause, specifying the columns to group by. Then, select the groups, as you only want one set of keys for each group.
Another easy option is to create a single distinct string.
var result = collection.DistinctBy(c => c.Field1 + "." + c.Field2 + "." + c.Field3);
var qry = (from cust in db.Customers
select new {cust.ID, cust.Name, cust.Region}).GroupBy(x => new { x.Name,x.Region}).select(z => z.OrderBy(i => i.cust).FirstOrDefault()).ToList();