* '''[[#Errors|BXDB0001]]''' is raised if {{Mono|$db}} references an XML node that is not stored in a database, or is no database fragment.
* '''[[XQuery Errors#BaseX Errors|BASX0003BXDB0002]]''' is raised if the addressed database cannot be opened.
The argument may also reference a BaseX-specific ''database fragment''. All XML fragments can be converted to database fragments by applying the [[XQuery_Update#transform|transform]] expression on an XML fragment: