A data model based on one-many relationships between aggregations of fixed numbers of data items, such an aggregation being termed a segment. A database record type comprises a number of segment types, arranged in a hierarchy, commencing with the root segment type; below the root segment type there is zero, one, or more segment types at the first level, with a similar structure below each of these first-level types at the second level, and so on. Thus each segment type except the root is dependent on a segment type at the immediately higher level. A database record instance comprises a single instance of the root segment type and zero, one, or more instances of each of its types at the first level. Corresponding to each of these first-level instances, there will be zero, one, or more instances of each of the appropriate second-level types, and so on. Only the root segment can have an independent existence.
IMS, an important database management system supplied by IBM, is based on and implements this data model.