I would like to get the list of albums (Distinct) which was sung by the artistId=1
I am very new to LINQ to SQL and do not know how to join multiple tables. Please see the database diagram below:
alt text http://a.imageshack.us/img155/8572/13690801.jpg
SingBy is the middle table between Track and Artist.
How could I achieve this?
var albums = from singer in artist
from sb in singby
from t in track
from a in album
where singer.artistId == 1 &&
sb.artistId == 1 &&
sb.trackId == t.trackId &&
a.albumId == track.albumId
select a;
I'm sure there must be a better way. You should look into creating Navigation Properties on your entities. Navigation Properties are like foreign keys.
Edit - corrected to get albums, not artists.
Now, I wrote the codes like the following and it works.
var albums = (from a in db.artists
where a.artistId == 1
join sb in db.singbies on a equals sb.artist
join t in db.tracks on sb.track equals t
join al in db.albums on t.album equals al
select al).Distinct();
return albums.ToList() as List<album>;
I tested the Chad's version and it works too. I would like to know which way is better and good for query optimization? Thanks all.
If you have all the foreign key relationship defined, you should be able to issue call like below:
dc.GetTable<Album>().Where(a => a.Track.Singby.ArtistId == 1).ToList();
This is relying on Linq to perform lazy load for Track and Singby automatically when required. Obviously this is not optimal to use when you have a large set of data in the db and performance is critical. You can chain the query with GroupBy or Distinct operation to return only the distinct set such as
dc.GetTable<Album>().Where(a => a.Track.Singby.ArtistId == 1).Distinct().ToList();
I would like to get the list of albums
(Distinct) which was sung by the
artistId=1
DBDataContext = new DBDataContext();
album[] = db.artists.Where(a => a.artistId == 1) /* Your artist */
.SelectMany(a => a.singbies) /* Check if `singby` converted to `singbies` */
.Select(sb => sb.track) /* The tracks */
.Select(t => t.album) /* The albums */
.GroupBy(al => al.albumId) /* Group by id */ /* "Distinct" for objects */
.Select(alG => alG.First()) /* Select first of each group */
.ToArray();
IEnumerable<Album> query =
from album in myDC.Albums
let artists =
from track in album.Tracks
from singBy in track.SingBys
select singBy.Artist
where artists.Any(artist => artist.ArtistId == 1)
select album;
List<int> Ids = dc.Albums.Where(a => a.Track.Singby.ArtistId == 1).Select(a=> a.albumId).Distinct().ToList();
List<Album> distinctAlbums = dc.Albums.Where(a => distinctAlbumIds.Contains(a.albumId)).ToList();
Hey TTCG, above is the simplest way to do it. This is because doing a Distinct on a List of objects won't do it based on the albumId.
Either you do it in two steps as above, or, you write your own Album Comparer which specifies uniqueness based on AlbumId and pass it to the Distinct call on a List.
NOTE:
The above will only work if you've defined the constraints in your DBML, but better still in your DB.
For best practices, always define your relationships IN THE DATABASE when using Linq to SQL, as Linq to SQL is not like EF, or NHibernate, in that is does not "abstract" your db, it simply reflects it. It's a tool for Data Driven Design, not Domain Driven, so define the relationships in the db.
Related
I have a database that contains 3 tables:
Phones
PhoneListings
PhoneConditions
PhoneListings has a FK from the Phones table(PhoneID), and a FK from the Phone Conditions table(conditionID)
I am working on a function that adds a Phone Listing to the user's cart, and returns all of the necessary information for the user. The phone make and model are contained in the PHONES table, and the details about the Condition are contained in the PhoneConditions table.
Currently I am using 3 queries to obtain all the neccesary information. Is there a way to combine all of this into one query?
public ActionResult phoneAdd(int listingID, int qty)
{
ShoppingBasket myBasket = new ShoppingBasket();
string BasketID = myBasket.GetBasketID(this.HttpContext);
var PhoneListingQuery = (from x in myDB.phoneListings
where x.phonelistingID == listingID
select x).Single();
var PhoneCondition = myDB.phoneConditions
.Where(x => x.conditionID == PhoneListingQuery.phonelistingID).Single();
var PhoneDataQuery = (from ph in myDB.Phones
where ph.PhoneID == PhoneListingQuery.phonePageID
select ph).SingleOrDefault();
}
You could project the result into an anonymous class, or a Tuple, or even a custom shaped entity in a single line, however the overall database performance might not be any better:
var phoneObjects = myDB.phoneListings
.Where(pl => pl.phonelistingID == listingID)
.Select(pl => new
{
PhoneListingQuery = pl,
PhoneCondition = myDB.phoneConditions
.Single(pc => pc.conditionID == pl.phonelistingID),
PhoneDataQuery = myDB.Phones
.SingleOrDefault(ph => ph.PhoneID == pl.phonePageID)
})
.Single();
// Access phoneObjects.PhoneListingQuery / PhoneCondition / PhoneDataQuery as needed
There are also slightly more compact overloads of the LINQ Single and SingleOrDefault extensions which take a predicate as a parameter, which will help reduce the code slightly.
Edit
As an alternative to multiple retrievals from the ORM DbContext, or doing explicit manual Joins, if you set up navigation relationships between entities in your model via the navigable join keys (usually the Foreign Keys in the underlying tables), you can specify the depth of fetch with an eager load, using Include:
var phoneListingWithAssociations = myDB.phoneListings
.Include(pl => pl.PhoneConditions)
.Include(pl => pl.Phones)
.Single(pl => pl.phonelistingID == listingID);
Which will return the entity graph in phoneListingWithAssociations
(Assuming foreign keys PhoneListing.phonePageID => Phones.phoneId and
PhoneCondition.conditionID => PhoneListing.phonelistingID)
You should be able to pull it all in one query with join, I think.
But as pointed out you might not achieve alot of speed from this, as you are just picking the first match and then moving on, not really doing any inner comparisons.
If you know there exist atleast one data point in each table then you might aswell pull all at the same time. if not then waiting with the "sub queries" is nice as done by StuartLC.
var Phone = (from a in myDB.phoneListings
join b in myDB.phoneConditions on a.phonelistingID equals b.conditionID
join c in ph in myDB.Phones on a.phonePageID equals c.PhoneID
where
a.phonelistingID == listingID
select new {
Listing = a,
Condition = b,
Data = c
}).FirstOrDefault();
FirstOrDefault because single throws error if there exists more than one element.
I have three tables, which two of them are in many to many relationship.
Picture:
This is the data in middle mm table:
Edit:
Got until here, I get proper 4 rows back, but they are all the same result(I know I need 4 rows back, but there are different results)
return this._mediaBugEntityDB.LotteryOffers
.Find(lotteryOfferId).LotteryDrawDates
.Join(this._mediaBugEntityDB.Lotteries, ldd => ldd.LotteryId, lot => lot.Id, (ldd, lot) =>
new Lottery
{
Name = lot.Name,
CreatedBy = lot.CreatedBy,
ModifiedOn = lot.ModifiedOn
}).AsQueryable();
My question is, how can I retrieve all the Lotteries via many to many table WHERE I have LotteryOfferId given only?
What I want to achieve is to get data from lottery table by LotteryDrawDateId.
First I use LotteryOfferId to get DrawDates from middle table, and by middle table I get drawDateIds to use them in LotteryDrawDate table. From that table I need to retreive Lottey table by LotteryId in LotteryDrawDate table.
I gain this by normal SQL(LotteryOffersLotteryDrawDates is middle table in DB, not seen in model):
select
Name, Lotteries.CreatedBy, Lotteries.ModifiedOn, count(Lotteries.Id)
as TotalDrawDates from Lotteries join LotteryDrawDates on Lotteries.Id
= LotteryDrawDates.LotteryId join LotteryOffersLotteryDrawDates on LotteryDrawDates.Id =
LotteryOffersLotteryDrawDates.LotteryDrawDate_Id
where LotteryOffersLotteryDrawDates.LotteryOffer_Id = 19 group by
Name, Lotteries.CreatedBy, Lotteries.ModifiedOn
But Linq is different story :P
I would like to do this with lambda expressions.
Thanks
db.LotteryOffer.Where(lo => lo.Id == <lotteryOfferId>)
.SelectMany(lo => lo.LotteryDrawDates)
.Select( ldd => ldd.Lottery )
.GroupBy( l => new { l.Name, l.CreatedBy, l.ModifiedOn } )
.Select( g => new
{
g.Key.Name,
g.Key.CreatedBy,
g.Key.ModifiedOn,
TotalDrawDates = g.Count()
} );
You can do this:
var query = from lo in this._mediaBugEntityDB.LotteryOffers
where lo.lotteryOfferId == lotteryOfferId
from ld in lo.LotteryDrawDates
group ld by ld.Lottery into grp
select grp.Key;
I do this in query syntax, because (in my opinion) it is easier to see what happens. The main point is the grouping by Lottery, because you get a number of LotteryDrawDates any of which can have the same Lottery.
If you want to display the counts of LotteryDrawDates per Lottery it's better to take a different approach:
from lot in this._mediaBugEntityDB.Lotteries.Include(x => x.LotteryDrawDates)
where lot.LotteryDrawDates
.Any(ld => ld.LotteryDrawDates
.Any(lo => lo.lotteryOfferId == lotteryOfferId))
select lot
Now you get Lottery objects with their LotteryDrawDates collections loaded, so afterwards you can access lottery.LotteryDrawDates.Count() without lazy loading exceptions.
I have a sql statement like this:
DECLARE #destinations table(destinationId int)
INSERT INTO #destinations
VALUES (414),(416)
SELECT *
FROM GroupOrder grp (NOLOCK)
JOIN DestinationGroupItem destItem (NOLOCK)
ON destItem.GroupOrderId = grp.GroupOrderId
JOIN #destinations dests
ON destItem.DestinationId = dests.destinationId
WHERE OrderId = 5662
I am using entity framework and I am having a hard time getting this query into Linq. (The only reason I wrote the query above was to help me conceptualize what I was looking for.)
I have an IQueryable of GroupOrder entities and a List of integers that are my destinations.
After looking at this I realize that I can probably just do two joins (like my SQL query) and get to what I want.
But it seems a bit odd to do that because a GroupOrder object already has a list of DestinationGroupItem objects on it.
I am a bit confused how to use the Navigation property on the GroupOrder when I have an IQueryable listing of GroupOrders.
Also, if possible, I would like to do this in one trip to the database. (I think I could do a few foreach loops to get this done, but it would not be as efficient as a single IQueryable run to the database.)
NOTE: I prefer fluent linq syntax over the query linq syntax. But beggars can't be choosers so I will take whatever I can get.
If you already have the DestinationGroupItem as a Navigation-property, then you already have your SQL-JOIN equivalent - example. Load the related entities with Include. Use List's Contains extension method to see if the desired DestinationId(s) is(are) hit:
var destinations = new List<int> { 414, 416 };
var query = from order in GroupOrder.Include(o => o.DestinationGroupItem) // this is the join via the navigation property
where order.OrderId == 5662 && destinations.Contain(order.DestinationGroupItem.DestinationId)
select order;
// OR
var query = dataContext.GroupOrder
.Include(o => o.DestinationGroupItem)
.Where(order => order.OrderId == 5662 && destinations.Contain(order.DestinationGroupItem.DestinationId));
I have two IEnumerables:
IEnumerable<ThisEmployee> thisEmployees;
IEnumerable<ThatEmployee> thatEmployees;
They are populated from 2 separate contexts. ThisEmployee and ThatEmployee are not matching types. They don't share anything similar apart from an EmployeeNumber property.
I want to get all ThatEmployee.Notes for any employee in thatEmployees that has a matching EmployeeNumber in thisEmployees.
I can't for the life of me work out how.
Your collections come from different contexts so get ids of employees first in linq-to-objects:
var ids = from e1 in thatEmployees
join e2 in thisEmployees on e1.EmployeeNumber equals e2.EmployeeNumber
select e1.Id;
Now use ids to get Notes from the database in single query
var notes = from n in context.Notes
where ids.Contains(n.Employee.Id)
select n;
Since its in two different contexts try using ToList to get all objects. Then using Linq to Objects u can use Where(r => thisEmployees.Any(s => s.EmployeeNumber == r.EmployeeNumber)). Not sure if i understood u correctly :)
How about something like:
var notes = thatEmployees
.Join(thisEmployees,
ta => ta.EmployeeNumber,
ti => ti.EmployeeNumber,
(ta, ti) => ta.Notes)
i have some Linq to Entity code like so:
var tablearows = Context.TableB.Include("TableA").Where(c => c.TableBID == 1).Select(c => c.TableA).ToList();
So i'm returning the results of TableA with TableB.TableBID = 1
That's all good
Now how can I sort TableA by one of its column? There is a many to many relation ship between the two tables
I tried various ways with no look, for example
var tablearows = Context.TableB.Include("TableA").Where(c => c.TableBID == 1).Select(c => c.TableA).OrderBy(p => p.ColumnToSort).ToList();
In the above case when i type "p." i don't have access to the columns from TableA, presumably because it's a collection of TableA objects, not a single row
How about using SelectMany instead of Select :
var tablearows = Context.TableB.Include("TableB")
.Where(c => c.TableBID == 1)
.SelectMany(c => c.TableA)
.OrderBy(p => p.ColumnToSort)
.ToList();
EDIT :
The expression below returns collection of TableAs -every element of the collection is an instance of TableA collection not TableA instance- (that's why you can't get the properties of the TableA) :
var tablearows = Context.TableB.Include("TableB")
.Where(c => c.TableBID == 1)
.Select(c => c.TableA);
If we turn the Select to SelectMany, we get the result as one concatenated collection that includes elements :
var tablearows = Context.TableB.Include("TableB")
.Where(c => c.TableBID == 1)
.SelectMany(c => c.TableA);
Okay, so now I've taken on board that there's a many to many relationship, I think Canavar is right - you want a SelectMany.
Again, that's easier to see in a query expression:
var tableARows = from rowB in Context.TableB.Include("TableA")
where rowB.TableBID == 1
from rowA in rowB.TableA
orderby rowA.ColumnToSort
select rowA;
The reason it didn't work is that you've got a different result type. Previously, you were getting a type like:
List<EntitySet<TableA>>
(I don't know the exact type as I'm not a LINQ to Entities guy, but it would be something like that.)
Now we've flattened all those TableA rows into a single list:
List<TableA>
Now you can't order a sequence of sets by a single column within a row - but you can order a sequence of rows by a column. So basically your intuition in the question was right when you said "presumably because it's a collection of TableA objects, not a single row" - but it wasn't quite clear what you mean by "it".
Now, is that flattening actually appropriate for you? It means you no longer know which B contributed any particular A. Is there only actually one B involved here, so it doesn't matter? If so, there's another option which may even perform better (I really don't know, but you might like to look at the SQL generated in each case and profile it):
var tableARows = Context.TableB.Include("TableA")
.Where(b => b.TableBID == 1)
.Single()
.TableA.OrderBy(a => a.ColumnToSort)
.ToList();
Note that this will fail (or at least would in LINQ to Objects; I don't know exactly what will happen in entities) if there isn't a row in table B with an ID of 1. Basically it selects the single row, then selects all As associated with that row, and orders them.