Data models

Most large software systems make use of a large database of information. In some
cases, this database is independent of the software system. In others, it is created
for the system being developed. An important pmt of systems modelling is defining the logical fonn of the data processed by the system. These are sometimes called
semantic data models.
The most widely used data modelling technique is Entity-Relation-Attribute modelling
(ERA modelling), which shows the data entities, their associated attributes
and the relations between these entities. This approach to modelling was first proposed
in the mid-1970s by Chen (Chen, 1976); several variants have been developed
since then (Codd, 1979; Hammer and Mcleod, 1981; Hull and King, 1987),
all with the same basic fonn.