XML data is semistructured and hierarchical, unlike the data in relational database tables. Therefore, you cannot search XML data with SQL queries unless you first relationalize the XML data (that is, put it in a relational database table).
Not all XML data can be relationalized; therefore, the XMLParser function constrains the relationships in the extracted data to grandparent/parent/child, parent/child, ancestor, and sibling relationships. The function lets you specify these constraints and the output table schema.