FROM component_reference [[LEFT | INNER] JOIN component_reference] ...
The FROM clause indicates the component or components from which to retrieve records.
If you name more than one component, you are performing a join.
For each table specified, you can optionally specify an alias.
The default join type is LEFT JOIN. This join can be indicated by the use of either 'LEFT JOIN' clause or simply ',', hence the following queries are equal:
SELECT u.*, p.* FROM User u LEFT JOIN u.Phonenumber
SELECT u.*, p.* FROM User u, u.Phonenumber p
INNER JOIN produces a Cartesian product between two specified components (that is, each and every record in the first component is joined to each and every record in the second component).
So basically INNER JOIN can be used when you want to efficiently fetch for example all users which have one or more phonenumbers.
SELECT u.*, p.* FROM User u INNER JOIN u.Phonenumber p