Databases Reference
In-Depth Information
Relational Basics
The relational database uses the concept of linked two-dimensional tables consisting of
rows and columns, as shown in
Figure 1-2
. Unlike the hierarchical approach, no pre‐
determined relationship exists between distinct tables. This means that data needed to
link together the different areas of the network or hierarchical model need not be de‐
fined. Because relational users don't need to understand the representation of data in
storage to retrieve it (many such business users create ad hoc queries), ease of access
combined with improved flexibility to change data models to adapt to changing business
needs and helped popularize the relational model.
Figure 1-2. Relational model with two tables
Relational programming is nonprocedural and operates on a set of rows at a time. In a
master-detail relationship between tables, there can be one or many detail rows for each
individual master row, yet the statements used to access, insert, or modify the data
simply describe the set of results. In many early relational databases, data access required
the use of procedural languages that worked one record at a time. Because of this set
orientation, programs access more than one record in a relational database more easily.
Relational databases can be used more productively to extract value from large groups
of data.
The contents of the rows in
Figure 1-2
are sometimes referred to as
records
. A column
within a row is referred to as a
field
. Tables are stored in a database
schema
, which is a
logical organizational unit within the database. Other logical structures in the schema
often include the following:
Views
Provide a single view of data derived from one or more tables or views. The view
is an alternative interface to the data, which is stored in the underlying table(s) that
makes up the view.
