Consider an HABTM example of customer - subscription - magazine with default setting for joinType. We need to be able to retrieve customers with no subscriptions and magazines with no subscriptions without having to change the joinType to outer.

Therefore the INNER JOIN needs to be grouped in parentheses before LEFT OUTER JOINing to customer or magazine.

Here's a good explanation:
http://www.bennadel.com/blog/1059-grouping-join-clauses-in-sql.htm

Possible implementation:

• We only need to nest when both inner and outer joins are used (so do a check for that before starting to nest the FROM clause).
• When joinType is "inner" we need to nest it (in other words subscriptions will, by default, be inner joined to magazines inside parentheses before being left outer joined to customers).
• We need a test case where there are multiple inner joins nested (the one at the right in the "include" string should be the most deeply nested one).