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LINQ: Is there a way to combine these queries into one?

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.

LINQ Groupby query on a single column

I have a situation where I have a Job which has multiple tests which run at specific intervals. A job run generates a unique TestRunId which is a GUID, which is used to reference multiple results, basically grouping a particular run with a unique RunId(GUID).
Now the problem is that I need to select unique runs which have been generated, but my LINQ query picks up every run.
I tried something like this
var testRunIds = ((from tests in context.testresults
where tests.JobId == jobId
select new
{
tests.TestRunId
}).GroupBy(t=>t.TestRunId).OrderBy(t=>t.Key).Skip((pagenum - 1) * pagesize).Take(pagesize)).ToList();
But as I said, this query picks up each and every testResult. Not sure how I do this now. I tried Distinct(), but that too didnt work. Sample data below.
Thanks
I believe the problem is that I have multiple TestRunId values as its essentially a grouping. Inorder to achieve what I need, I tried using (got using Linqer)
from Queries in db.TestResult
where
Queries.JobId == 1
group Queries by new {
Queries.TestRunId,
Queries.StartTime,
Queries.EndTime
} into g
orderby
g.Key.TestRunId
select new {
_ID = (int?)g.Max(p => p.Id),
g.Key.TestRunId,
g.Key.StartTime,
g.Key.EndTime
}
But this works only for MSSQL datasource which is essentially a
SELECT max(id)[ ID],
TestRunId,
StartTime,
Endtime
FROM dbo.query where jobid = 1 group by TestRunId,StartTime,Endtime order by StartTime;
But what I need is
SELECT TestRunId,StartTime,Endtime FROM testresult where jobid = 1 group by TestRunId order by StartTime;
for MySQL.

Try this:-
var jobs = context.testresults;
var query2 = jobs.Where(x => x.TestID == 1).OrderBy(x => x.StartTime).Select(x => x.TestRunID).Distinct();
Working Fiddle.

I think you're possibly looking for this:
var testRunIds = context.testresults.Where(t => t.JobId == jobId).OrderBy(t => t.starttime)
.Select(t => t.TestRunId).Distinct().Skip((pagenum - 1) * pagesize).Take(pagesize)
.ToList();
Do the filtering and ordering first, then select the single field needed, then use Distinct() for uniqueness, then skip/take as required. Selecting the single field first then attempting to order or filter on other fields in the table won't work as those fields are no longer part of the query.

Thanks for helping me out. I managed to do this in a two step process.
var testRunIds = (from tests in context.testresults
where tests.JobId == jobId
select new
{
tests.TestRunId,
tests.StartTime
}).OrderBy(x => x.StartTime).Skip((pagenum - 1) * pagesize).Take(pagesize).GroupBy(x=>x.TestRunId).ToList();
var resultData = testRunIds.Select(testRunId => (context.testresults.Where(
items => items.TestRunId == testRunId.Key)).FirstOrDefault()).ToList();

How to execute query to select multiple attributes subject to several where clauses across two tables

I have two tables from which I want to select data from:
Document_Data
Document_info
I want to execute the following query :
SELECT DISTINCT Document_Data.DOC_CLASS, TITLE FROM Document_info,Document_Data WHERE (((DOC_STATUS = '1') AND (PORTAL = 'First Page'))) AND (Document_info.DOC_NUMBER = Document_Data.DOC_NUMBER AND Document_info.REVISION = Document_Data.REVISION AND STATUS = 'CURRENT' AND Document_Data.DOC_CLASS = 'MESSAGE')
Can anyone give me info on how to execute the following query using Linq?

I have made a few assumptions since your query did leave off a few table names. I assumed that STATUS was on the Document_data table and DOC_STATUS was on the Document_info table. If its any different, it shouldn't be hard to modify this query to work.
DbContext is your entity framework context or wherever your store your db collections.
dbContext.Document_info.Where(i => i.DOC_STATUS == "1" && i.PORTAL == "First Page")
.Join(dbContext.Document_data.Where(d => d.DOC_CLASS == "MESSAGE" && d.STATUS == "CURRENT"),
i => new { i.REVISION, i.DOC_NUMBER }, //Document_info
d => new { d.REVISION, d.DOC_NUMBER }, //Document_data
(i, d) => new { d.DOC_CLASS, i.TITLE }) //(Document_info, Document_data)
.Distinct()
.ToList();
The way this works is that it first filters the document_info table to what you wanted from there. It then joins it with a filtered Document_data table on a composite "key" made up of REVISION and DOC_NUMBER. After that, it runs the Distinct and executes the whole query with a ToList.
The above should compile to valid SQL (at least it would using the MySQL connector...I haven't tried anything like that with MSSQL, but I assume that since the MSSQL one works better than MySQL so it would make sense that it would work there too). This particular query would come out to be a little convoluted, however, and might not work very optimally unless you have some foreign keys defined on REVISION and DOC_NUMBER.
I would note that your query will only return things where d.DOC_CLASS == "MESSAGE" and so your results will be quite repetitious.

c# only show numbers after decimal point with linq

I have a linq query that executes successfully, one of the columns returned is a decimal type that is used to represent prices in pounds and pence (there will never be any negative values)
I want to be able to strip out the pounds and pence into separate Properties of my projection, however when using functionality such as
var result= from j in context.Products
select
new{
Price = t.Price,
PricePounds = Math.Truncate(t.Price)
};
I get an error that Math.truncate is not supported as it cannot be translated into a store expression. How can I get the pounds value from this query?

If you don't need to do anything else in the database after that, the simplest approach is just to perform the truncation client-side:
var query = context.Products
.AsEnumerable() // Everything from here is LINQ to Objects
.Select(p => new {
p.Price,
PricePounds = Math.Truncate(p.Price)
});
Note that you might also want to just cast to int - and that might be supported in EF already.
EDIT: As noted in comments, you may want to perform a projection first, e.g.
var query = context.Products
.Select(p => new { p.Price, p.SomethingElse })
.AsEnumerable() // Everything from here is LINQ to Objects
.Select(p => new {
p.Price,
PricePounds = Math.Truncate(p.Price),
p.SomethingElse
});
(Where SomethingElse is another property you're interested in, as an example - I doubt that you only want the price.)
This will avoid the entire entity being fetched when you only want a few properties.

You may try:
var result= from j in context.Products
select
new {
Price = t.Price,
PricePounds = EntityFunctions.Truncate(t.Price, 0)
};
The case is Math.Truncate cannot be translated into SQL where EntityFunctions.Truncate should be.

Linking Multiple Tables in LINQ to SQL

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.