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SharePoint and List Joins with CAML

This came to me as an issue when I started migrating one project from SharePoint 2010 to SharePoint 2013. The code uses LINQ to SharePoint and it was not written in a clever way. We decide to use CAML queries and optimize the code.

The scenario: Create gridview that gathers information from three(or more) lists.
The problem: How to create the CAML query joins and projected fields correctly.


Option 1: Get generated CAML query from LINQ to SharePoint code
This doesn’t work. I wrote the LINQ to SharePoint code connected the three lists. When I executed the code the following error was thrown – “The query uses unsupported elements, such as references to more than one list, or the projection of a complete entity by using EntityRef/EntitySet.Here is some explanation about it and why this can’t be used.

Option 2: Write the query yourself
I found this article in MSDN. Read it !
The second part in it is exactly my scenario. But it takes me a lot of time to understand how exactly to write this in server-side code. So here is a working code of the described scenario:

And yes, this works but what if we want to get more columns. Here is same scenario but more complicated.

I have added new colums and added new projected fields in the query.
The following code get all the additional data:

Some important points:
1. Only the Order’s list columns can be used with their real internal names.
2. The other columns are all projected and there are limitaton of the column's type. For example the 'Contact Person' column in my scenario can't be a Lookup or Person and Group.
3. The type of the projected field is always ‘Lookup
4. ShowField is the value of the internal names of the fields
5. The ListAlias and List in the Join statement are not the real names(or Urls) of the list. The next Join statement is equal to the above one:



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