| | 26 | Q: Is this correct? !ElementTree without pointers between separate trees does not seem to have a way to encode a directed acyclic graph. A general database schema is a DAG plus self-loops (references from a table to itself, so long relations among elements are not cyclic). (For instance, consider volunteers. They have components via pe_id. They also have references to zero or more elements of the volunteer skills table. Other volunteers point to those same skill records. Thus there are multiple roots -- the skills -- to the tree of volunteers. The same structure occurs in inventory, where catalog items are referenced by multiple order items, but order items are also components of orders. In these cases, there isn't a (clean) way to pick one root for a tree. If we decided to have an skill category table, then we would have diamond-shaped DAGs -- a volunteer could point to several skills, and those skills could point to a common category.) For output, this is not relevant because the records will have their primary keys and foreign keys available. It's only an issue when creating a collection of dag-structured data, as no actual keys have been assigned yet. This is not hard to overcome -- it just means adding placeholder keys to represent the linkage between records in separate ElementTrees. There are examples of DAG representations and algorithms -- a search for "xml directed acyclic graph" will turn them up. |
