The SQL repository allows applications to query repository items based on attributes of their attributes, sometimes referred to as subproperties.

Continuing the book example, subproperty querying means that you can write repository queries like:

Get me all the books that were written by authors living in NY

This query can be represented using RQL in a <query-items> tag, as follows:

<query-items item-descriptor="book">author.state="NY"</query-items>