Database Reference
In-Depth Information
base itself, especially in terms of the way data is collected and managed. This type of data-
base typically contains inconsistent data, erroneous data, duplicate data, redundant data,
incomplete entries, and old data that should have been purged from the database long ago.
Clearly, the only reason you'd analyze this type of database is to identify items that you
could incorporate into the new database. For example, you can extract individual pieces of
data from various sections of a form in the old database and transform them into fields in
the new database.
Legacy Databases
A
legacy database
is a database that has been in existence and in use for five years or
more. Mainframe databases typically fall into this category, as do older PC-based data-
bases. There are several reasons that “legacy” is used as part of the name for this type
of database. First, it suggests that the database has been around for a long time, possibly
longer than anyone can clearly remember. Second, the word
legacy
may mean that the in-
dividual who originally created the database either has shifted responsibilities within the
organization or is working for someone else and, thus, the database has become his or her
legacy to the organization. Third, the term implies the disturbing possibility that no single
individual completely understands the database structure or how it is implemented in the
RDBMS application program.
Mainframe legacy databases present some special problems in the analysis process. One
problemstemsfromthefactthatanumberofoldermainframe databases arebasedonhier-
archical or network database models. If neither you nor anyone in the organization has a
firm understanding of these models, it will take you some time to decipher the structure of
thedatabase. Inthiscase,you'llfinditveryhelpfultomakeprintoutsofthedataineachof
the database structures.
Even if a legacy database is based on the relational model, there's no particular guarantee
thatthestructureissound.Unfortunately,therearemanyinstancesinwhichthepeoplewho
created these databases didn't completely understand the concept of a relational database.
(Afteryouhavereadthisbook,youwon'tfallintothatgroup.)Theresultisthatmanyolder
databases have improper or inefficient structures.
Numerous PC-based legacy databases are improperly or inefficiently designed, too. Many
of them were originally developed and implemented in older nonrelational database man-
agement systems, which means that they could not take advantage of the benefits provided
bytherelational model. Twocharacteristics commonly associated withthese typesofdata-
bases are duplicate fields and redundant data. You'll learn later that this can cause serious
problems with data integrity.
