Question

I have a data tables with this type of setup.

Table A

AID     AName --------------------- 1       A_Name1 2       A_Name2 3       A_Name3 

Table B

BID     BName --------------------- 10      B_Name1 20      B_Name2 30      B_Name3 

Table C

AID     BID --------------------- 1       10 2       10 2       20 2       30 3       20 3       30 

I want to use LINQ to write a query that for a given ID of A give me the list of B that is not in table C.

So for AID of 1 I want BID of 20 & 30.

For AID of 3 I want BID of 10

For AID of 2 nothing returned anything returned.

I know the SQL would be

SELECT     B.BID FROM B LEFT OUTER JOIN C ON C.BID = B.BID AND A.AID = 1  -- AID would change based on which I was looking for, 1, 2, or 3 

What is the LINQ Equivalent?

Please assume I have objects that match the tables of A, B, and C accordingly.

Answer 1

For LinqToSql, database optimizer will handle this appropriately. Use the ‘Not Any’ pattern.

IQueryable<B> query =   db.BTable     .Where(b => !db.CTable       .Where(c=> c.AID == 1)       .Any(c => c.BID == b.BID)     ) 

For LinqToObjects, we don’t want to enumerate the CList for each item in BList. Do that enumeration one time.

List<int> BIDList = CList   .Where(c => c.AID == 1)   .Select(c => c.BID)   .ToList(); IEnumerable<B> query = BList   .Where(b => !BIDList.Contains(b.BID));