Function SQL join

Yes and no.

In fact there is no specific join function for SQL queries, such as the ones you can find on hibernate.
The framework bring a more simple and pragmatic approach :
Considering that joining tables is mostly used to retreive a name from an id through a foreign key, all is done in PHP scripts. Only main table is retrieved, and id->name conversion is done throught a devoted function highly using cached data. This way queries are kept simple and performance is assumed.

It may not fit all situations, but answers all need for Project'Or RIA.

One exception is the PlanningElement table : for higher performance, it has been de-normalized, copying required names in the table. The framework assumes the consistancy of these names.