Relationship queries are similar to SQL joins. However, you cannot perform arbitrary SQL joins. The relationship queries in SOQL must traverse a valid relationship path as defined in the rest of this section.
You can use relationship queries to return objects of one type based on criteria that applies to objects of another type, for example, “return all accounts created by Bob Jones and the contacts associated with those accounts.” There must be a parent-to-child or child-to-parent relationship connecting the objects. You can’t write arbitrary queries such as “return all accounts and users created by Bob Jones.”