• In simplest terms, a database is a collection of information. This collection
is stored in well-defined tables, or matrices.
• The rows in
a database table are used to describe similar items. The rows are referred to
as database records. In general, no two rows in a database table will be alike.
• The columns in a database table provide characteristics of the
records. These characteristics are called database fields. Each field contains
one specific piece of information. In defining a database field, you specify the
data type, assign a length, and describe other attributes.
•
Here is a simple database example:
In this database table, each record represents a single individual.
The fields (descriptors of the individuals) include an identification number (ID
No), Name, Date of Birth, Height, and Weight.
• Most databases use indexes to allow faster access to
the information in the database. Indexes are sorted lists that point to a particular
row in a table. In the example just seen, the ID No field could be used as an
index.
• A database using a single table is called a flat database.
Most databases are made up of many tables. When using multiple tables within a
database, these tables must have some common fields to allow cross-referencing
of the tables. The referral of one table to another via a common field is called
a relation. Such groupings of tables are called relational databases.
• In our first example, we will use a sample database
that comes with Visual Basic. This database (BIBLIO.MDB) is found in the main
Visual Basic directory (try c:\Program Files\Microsoft Visual Studio\VB98). It
is a database of books about computers. Let’s look at its relational structure.
The BIBLIO.MDB database is made up of four tables:
Authors Table (6246 Records, 3 Fields)
The Authors table consists of author identification
numbers, the author’s name, and the year born. The Publishers
table has information regarding book publishers. Some of the fields include an
identification number, the publisher name, and pertinent phone numbers. The Title
Author table correlates a book’s ISBN (a universal number assigned
to books) with an author’s identification number. And, the Titles
table has several fields describing each individual book, including title, ISBN,
and publisher identification
Note each table has two types of information: source
data and relational data. Source data is actual information,
such as titles and author names. Relational data are references to data in other
tables, such as Au_ID and PubID. In the Authors, Publishers and Title Author tables,
the first column is used as the table index. In the Titles table,
the ISBN value is the index.
• Using the relational data in the four tables, we should
be able to obtain a complete description of any book title in the database. Let’s
look at one example:
Here, the book in the Titles table, entitled
“Step-by-step dBase IV,” has an ISBN of 0-0280095-2-5 and a PubID
of 52. Taking the PubID into the Publishers table, determines
the book is published by McGraw-Hill and also allows us to access all other information
concerning the publisher. Using the ISBN in the Title Author
table provides us with the author identification (Au_ID) of 171, which, when used
in the Authors table, tells us the book’s author is Toby
Wraye.
• We can form alternate tables from a database’s
inherent tables. Such virtual tables, or logical views,
are made using queries of the database. A query is simply a request
for information from the database tables. As an example with the BIBLIO.MDB database,
using pre-defined query languages, we could ‘ask’ the database to
form a table of all authors and books published after 1992, or provide all author
names starting with B. We’ll look briefly at queries.
• Keeping track of all the information in a database is handled by a database management system (DBMS). They are used to create and maintain databases.
Examples of commercial DBMS programs are Microsoft Access, Microsoft FoxPro, Borland
Paradox, Borland dBase, and Claris FileMaker. We can also use Visual Basic to
develop a DBMS. Visual Basic shares the same ‘engine’ used by Microsoft
Access, known as the Jet engine. In this class, we will see how
to use Visual Basic to access data, display data, and perform some elementary
management operations.
