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Databases

tells one that one should give the value from the name col- browsed from remote locations. A person interested in
umn as one’s result. As shown before, the resulting cus- locating a book in a library can enter the book’s title,
tomer name is “Brown.” author, or subject, and a database query will be automati-
Queries can also be used to perform calculations: cally performed. Information on the desired book or list
of books will be returned to the person’s computer.

Query: What is the average price of our products?
SELECTING A DATABASE SYSTEM
A person or business seeking to purchase a DBMS for use
SQL solution: Select Avg (price)
From Products in managing a database should consider the following fac-
Result: 3.01
tors:
Relational: Virtually all major commercial DBMSs
SQL also provides statements that can be used to are relational, because the desirability of rela-
make changes to data in the database. For example, sup- tional databases is well-accepted in the database
pose one wanted to increase the price of one’s products by community.
3 percent. Then the following statement can be used:
SQL: In addition, because the ANSI has adopted
Update Products SQL as it standard for relational databases, the
Set price = 1.03 * price desired DBMS should support SQL.
Capacity: As noted above, DBMSs are designed for a
This statement will cause the price of every product in the
Products table to be increased by 3 percent. Note that it variety of environments. Some are designed to be
single-user systems, while others are designed for
does not matter whether one has 3 products, as shown in
the sample database, or 300,000 products. A single state- medium-sized businesses, while still others are
ment will update the prices of all products. Of course, if designed for large businesses. The system selected
one wants to change only the prices of selected products, should naturally be one that has been shown to
one can do that, too: be successful in and appropriate for the environ-
ment for which it is chosen.
Update Products
Disaster recovery capability: More sophisticated sys-
Set price = 1.03 * price
Where product_no = 33 tems are more capable of recovering from power
outages, computer hardware failure, and the like
This statement will change only the price of product than are the single-user systems. They use sophis-
number 33. SQL also provides statements to Insert new ticated logging and database locking facilities that
rows into tables and to Delete rows from tables. make such recovery possible. Often, these facili-
These queries show only a very small number of the ties are unnecessary for single-user systems.
capabilities of SQL. The Where clause can be used to
select rows based on where names are in the alphabet, SUMMARY
whether dates are before or after certain other dates, based
on averages, and based on many other conditions. Databases and DBMSs are central to modern business
information systems. Relational databases using SQL pro-
vide substantial logical power to help businesses make
SMALL AND LARGE DATABASES informed decisions based on their own data. Database sys-
Databases can be single-user or multiuser. A single-user tems can be small and handled by a single user, or they can
database exists on a single computer and is accessible only be large and available to multiple users. They are even
from that computer. Many single-user databases exist, and publicly available through the Internet. DBMSs can be
there are a number of commercial database manufacturers sophisticated and expensive, and consequently their pur-
that address this market. A multiuser database may exist
chase requires careful, informed consideration.
on a single machine, such as a mainframe or other power-
ful computer, or it may be distributed and exist on multi- SEE ALSO Information Technology; Software
ple computers. Multiuser databases are accessible from
multiple computers simultaneously. BIBLIOGRAPHY
With the rise of the Internet, many databases are About Databases Guide Site. http://databases.about.com
publicly accessible. For example, the holdings of univer- Dunham, Jeff (1998). Database performance tuning handbook.
sity libraries are maintained on databases that can be New York: McGraw-Hill.

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