The next level of the data model is the place where much detail is found. This level of the
data model is called the “data item set” (dis).
Each entity identified in the ERD has its own dis. Using the simple example shown in Fig.
7.3.3, there would be one dis for customer, another dis for order, another dis for product,
and yet another dis for shipment.
The dis contains keys and attributes, and the dis shows the organization of the data.
The symbol for a simple dis is seen in Fig. 7.3.4.
Fig. 7.3.4 A data item set—dis.The basic construct of a dis is a box. In the box are the elements of data that are closely
related and that belong together. The different lines between the groupings of data have
meaning. A downward-pointing line indicates multiple occurrences of data. A line to the
right indicates a different type of data.
As a simple example of a dis, consider the dis shown in Fig. 7.3.5.
Chapter 7.3: Data Modeling for the Structured Environment