SQL queries can be thought of as working against a database view of the workspace being searched (similar in principle to the two XML views described in 6.4 XML Mappings).
Note however, that it is entirely up to the implementation whether this database view directly reflects the underlying storage mechanisms of the repository (as it might in repositories that are actually database-backed). All that is required is that if SQL queries are supported, they behave as if they were running against the database view described below.