How to Join Multiple Columns with Linq To SQL

Today I had to create a Linq-To-SQL query which joined two tables. These tables had compound primary keys (table simplified for example):

Table Referrers

ServerID (PK)	ReferrerID (PK)	Value
1	1	My value 1
1	2	My value 2
1	3	My value 3

Table ReferrerInfo

ServerID (PK)	ReferrerID (PK)	Value
1	1	More info… 1
1	2	More info… 2
1	3	More Info… 3

So I wanted to join these together using a Linq-To-SQL query. The first thought that occurred to me was to use a join:

var referrers = from r in Referrers
               join ri in ReferrerInfo on r.ServerConnectionID equals ri.ServerConnectionID ..... ??????
               select r;

So, apparently in C# you cannot do multiple columns in your join. I have to join on both ServerConnectionID and ReferrerID.

This page: http://www.onedotnetway.com/linq-to-sql-join-on-multiple-conditions/ explains one method to do so, but unfortunately that Linq query only works in VB.

So, I ended up writing a Linq-To-SQL that just uses the old style of joining, by using the where clause!

var referrers = from r in Referrers
                from ri in Referrer_Info
                where r.ServerConnectionID == ri.ServerConnectionID &amp;&amp;
                   r.ReferrerID == ri.ReferrerID
                select r;

This successfully executes my multi-column join.

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This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Dave C Andrews

Software Developer (Senior) LBMC Technologies

United States

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I've been developing software since I was 9 years old, in BASIC on a Precomputer 2000. OK, that's a stretch, but hopefully some of the stuff that I learned from that little toy have helped my professional career.

I have a bachelors degree in computer science from MTSU. Currently, I am employed as a senior software developer for LBMC in Nashville, TN. We serve as consultant developers for clients across the southeastern United States, developing mostly in Microsoft .Net with Microsoft SQL Server.

I greatly enjoy my job and my hobbies lie in open source software and in teaching others how to be better programmers. I have a beautiful wife, 2 dogs (a dachshund and an olde-english bulldog) and a cat.

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A Technical Blog, originally posted at http://davethedev.com/2011/12/how-to-join-multiple-columns-with-linq-to-sql/

Today I had to create a Linq-To-SQL query which joined two tables. These tables had compound primary keys (table simplified for example): Table Referrers ServerID (PK) ReferrerID (PK) Value 1 1 My value 1 1 2 My value 2 1 3 My value 3 Table ReferrerInfo ServerID (PK) ReferrerID (PK) Value 1 1 More i

Type	Technical Blog
Licence	CPOL
First Posted	27 Jan 2012
Views	8,763
Bookmarked	2 times

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How to Join Multiple Columns with Linq To SQL

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