A hierarchical structure of records, in which

1. (a) a record at level i holds data that is common to a set of records at level i + 1 and

2. (b) starting from the higher-level record, it is possible to access the set of lower-level records.

Any record may only ‘own’ one set of lower-level records, and may only be a member of one such set. A data hierarchy may reflect ‘real-world’ hierarchical relationships, or may be a user view provided by a DBMS to facilitate a purpose or activity.