Linq - How to do two select queries?

Question

SELECT userid FROM userTable 
WHERE userid in (select writeuserid FROM boardTable)

C# LINQ expressions, how to use a query?

I have been using EF4. userTable, boardTable is connected to the DbContext.

Your question isn't clear at all. And more important, _did you tried anything so far_? You can read [FAQ] and [ask] — Soner Gönül, Apr 01 '13 at 07:59
http://stackoverflow.com/questions/37324/what-is-the-syntax-for-an-inner-join-in-linq-to-sql — Shani Elharrar, Apr 01 '13 at 08:00
Do you have a model? For example, is userTable represented by a class? — idipous, Apr 01 '13 at 08:04
check this link http://stackoverflow.com/questions/418609/how-to-do-subquery-in-linq — Anton Semenov, Apr 01 '13 at 08:04

score 4 · Answer 1 · answered Apr 01 '13 at 08:00

4

Why not have two different LINQ queries, so that your inner query doesn't execute for each iteration of the outer query.

var query1 = (from t in dbContext.boardTable
            select t.writeuserid).ToArray();

var query2 = from r in dbContext.userTable
             where query1.Contains(r.userid)
             select r.userid;

If your situation is as simple as in the question then you cause join in linq

answered Apr 01 '13 at 08:00

Habib

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http://stackoverflow.com/questions/527819/linq-sub-select or check this one too – isioutis Apr 01 '13 at 08:02
+1 just for spend more effort than OP to solve this problem.. – Soner Gönül Apr 01 '13 at 08:03
Thank you for an answer. – user2214027 Apr 01 '13 at 08:06
Using ToArray() may be a bit dangerous if the amount of data is very large, since it will cause EF to materialize the entire array. A join may be more suited for that purpose. – Pablo Romeo Apr 01 '13 at 08:06
2

@PabloRomeo, actually I wasn't sure about the exact requirement of the OP, so presented him/her with the above option, also added a link for joins in linq – Habib Apr 01 '13 at 08:12

score 1 · Answer 2 · answered Apr 01 '13 at 08:03

1

Assume in here you use Entity FrameWork, so you can use Join to get the result, below is to use the lambda expression:

var result = dbContext.Users.Join(dbContext.Boards, 
                                  user => user.UserId, 
                                  board => board.WriteUserId,
                                  (u, b) => u.UserId);

answered Apr 01 '13 at 08:03

cuongle

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A little bit explanation would be great for OP, IMO.. – Soner Gönül Apr 01 '13 at 08:04

score 1 · Answer 3 · answered Apr 01 '13 at 08:06

1

why not using join?

var result = (from u in dbcontext.userTable 
join u1 in dbcontext.boardTable on u.userid equals u1.writeuserid
select u.userid).FirstOrDefault();

if (result != null)
 // do anything else
else
 // user not exists

answered Apr 01 '13 at 08:06

eRiCk

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Linq - How to do two select queries?

3 Answers3