FREE online courses on Information Technology - Chapter 7 IT – DATABASE
MANAGEMENT - DATABASES IN SYSTEMS DESIGN
It should be apparent at this point that one of the major design task in
building an information system is determining the contents and structure of a
database. The type of retrieval and reporting required by users and the
availability of input determine what data to store. However, it is a very
complex task to specify these data, group them into records, and establish data
structures for a system.
A data model is useful for a number of reasons. First, it helps us understand
the relationships among different components in a systems design. Data models
show users more clearly how a system will function. Users are very concerned
about data and information. They want to know if there will be adequate data
available to perform their jobs.
The most common type of data model is the entity-relationship (ER) diagram. The
ER diagram is easy for a user to follow and serves as an excellent
communications vehicle. The ER diagram consists of object types and
relationships. In Figure (a) below we see an example of two objects linked by a
relationship: A customer purchases a product. The two entities here are
“customer” and “product”; the relationship is “purchase”. Entities analysts like
to use a simple, straight line between entities and label the line with the
relationship, (as shown in Figure (b) below), though certain more-complex
relationships cannot be modeled in this manner.
Figures to be inserted
Figure: (a) An entity relationship (ER) diagram. (b) An alternative ER diagram.
Entities also have attributes, which are the fields we would
include in a file record. A product number, size, description, price, cost, and
so on. The ER diagram, then, can be used to show relationships while the
conventional listing of the file contents contains the attributes of entities.
