However, today??™s modern Relational
Database Management Systems (RDBMS), such as MySQL, PostgreSQL, SQL Server, Oracle, DB2,
and others, have extended this basic role by adding the capability to store and manage relational
data. This is a concept that deserves some attention.
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C H A P T E R 5
So what does relational data mean? It??™s easy to see that every piece of data ever written in
a real-world database is somehow related to some already existing information. Products are
related to categories and departments; orders are related to products and customers, and so
on. A relational database keeps its information stored in data tables but is also aware of the
relationships between the tables.
These related tables form the relational database, which becomes an object with a significance
of its own, rather than simply being a group of unrelated data tables. It is said that data
becomes information only when we give significance to it, and establishing relations with other
pieces of data is an ideal means of doing so.
Look at the product catalog to see what pieces of data it needs and how you can transform
this data into information. For the product catalog, you??™ll need at least three data tables: one
for departments, one for categories, and one for products. It??™s important to note that physically
each data table is an independent database object, even if logically it??™s part of a larger entity??”
in other words, even though we say that a category contains products, the table that contains
the products is not inside the table that contains categories.
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