A parent-child hierarchy is a simple, two-level hierarchy where the child element has an attribute containing the parent element. This hierarchy type is somewhat limited (to two levels, for example), though it does have some use in specific circumstances.

You might consider that an invoice having several lines is a parent-child hierarchy, where the invoice ID is the parent and the line item is the child, for example.

A level hierarchy, as its name suggests, does not limit the hierarchy to only two levels, making it much more flexible in terms of how many levels of data you can represent with one. You can define freely the hierarchy levels, as long as each child node has the parent node as an attribute. This allows us to create several different levels, where one node's child...