It is currently not possible to perform left outer join using Advanced Find, even though that capability exists in CRM2015. For e.g. if you want to get a list of contacts, that don’t have an invoice, you cannot do it using an ad-hoc Advanced Find.
This limitation is usually overcome by creating a system view that does a regular inner join, and then updating the view’s fetchxml using a tool like FXB or even editing the customisation file directly. There is also the rollup field approach described in http://crmtipoftheday.com/2015/04/27/not-in-for-the-rest-of-us/. But if you are going to do an ad-hoc advanced find query, you really don’t want to create one system view for each entity, for which you want to do an outer join.
I present to you a solution for this problem: AdvancedQueryOuterJoin
This is a plugin that runs on the pre-stage of RetrieveMultiple, and alters the fetchxml with the correct left outer join condition. This means you can do an Advanced Find Query like this one below.
The above query is for Account, who have child contacts, but don’t have any opportunities. When the plugin sees a link entity in the query, with a null primary key condition, it replaces that condition with a left outer join on the parent entity. The code itself is very simple, and you can have a look at the github repo to understand how this works.
You can download the solution from https://github.com/rajyraman/AdvancedQueryOuterJoin/releases. Download the solution file that works best for your CRM instance i.e. 7.0 or 7.1. Please also use the github issues area, for entering details about any problems that you encounter.
EDIT (01/06/15): Using this plugin will not correct the fetchxml you download by clicking the Download Fetch XML in the Advanced Find window. If you are going to run your downloaded query in FXB, make sure you change the Output type to Grid.
If you use FetchResult, you won’t see any results, as the plugin would not have executed in the background.
EDIT (02/10/15): Refer to the newer post https://dreamingincrm.com/2015/09/14/bookmarklet-advanced-find-outer-join-a-k-a-not-in-condition/ for doing this using bookmarklet.