Database Reference
In-Depth Information
or fields in a table is completely immaterial, and each record in the table is identified by a
field that contains a unique value. These are the two characteristics of a relational database
that allow the data to exist independent of the way it is physically stored in the computer.
As such, a user isn't required to know the physical location of a record in order to retrieve
its data. This is unlike the hierarchical and network database models in which knowing the
layout of the structures is crucial to retrieving data.
The relational model categorizes relationships as one-to-one, one-to-many, and many-to-
many. (These relationships are covered in detail in
Chapter 10
,
“
Table Relationships
.”) A
relationship between a pair of tables is established implicitly through matching values of a
shared field. In
Figure 1.5
, for example, the CLIENTS and AGENTS tables are related via
an Agent ID field; a specific client is associated with an agent through a matching Agent
ID. Likewise, the ENTERTAINERS and ENGAGEMENTS tables are related via an En-
tertainer ID; a record in the ENTERTAINERS table can be associated with a record in the
ENGAGEMENTS table through matching Entertainer IDs.
