FREE online courses on Information Technology - Chapter 7 IT – DATABASE
MANAGEMENT - Benefits of the Relational Model
The relational model is the dominant structure for vendors
writing DBMS. The underlying concept of a
relational file system is very simple: Data are organized in two-dimensional
tables. Such tables are easy for a user to develop and understand. One virtue of
this type of structure is that it can be described mathematically, a most
difficult task for other types of data structures. The name is derived from the
fact that each table represents a
relation.
Because different users see different sets of data and
different relationships among them, it is necessary to extract subsets of the
table columns for some users and to join tables together to form larger tables
for others. The mathematics provides the basis for extracting some columns from
the tables and for joining various columns.
Relational database management systems have many advantages.
Most DBMSs for personal computers are based on the relational model because it
is relatively easy for users to understand.
IBM developed
Structured Query Language (SQL) a number of years ago. The language was
first proposed as a retrieval
language for users, but it is difficult to use, so few users are likely to adopt
it. It is important to note that SQL is the query language for IMB's mainframe
relational database management package, DB2. There is an ANSI standard for SQL,
and this language is being adopted by the major DBMS vendors as one way to
interact with their systems. SQL also offers a mechanism for universal database
access. For example, suppose that the DBMS you are using translates the query
language you enter into SQL commands. It could then retrieve data on a different
system, so long as they both used the same SQL dialect.
1.
The basic structure of an SQL expression has three parts. The select clause lists the attributes desired in answer to the query.
2.
The
form clause is a list of relations or
tables that the query language processor should consult in filling the request.
As an example, consider the
following SQL expression taken from Korth and Silberschatz (1986):
Select branch – name
From deposit
This is an SQL expression to obtain
a list of all branch names from a bank table (branch-name) containing data about
branches and customers. One might find all customers having an account in the
Midtown branch with the following expression:
Select customer – name
From deposit
Where branch-name = “Midtown”)
SQL expression can become
complicated as we qualify retrieval requests:
(Select customer-name)
form deposit
where branch-name = “Midtown”
intersect
(select customer-name)
form borrow
where branch-name = “Midtown”
The above query produces a table of all customers who have
both a loar (form the borrow table) and an account in the Midtown branch.
There has been a proliferation of database management systems for all types of
computers and SQL appears as the one common thread. Various vendors are
designing their DBMS packages to translate queries using the package's interface
into SQL commands to query a remote database. Why would a user be interested in
such a feature?
Suppose you are working with Paradox, a PC database system, and want to access
data located on an IBM mainframe in a DB2 database. You would like to enter
Paradox queries and not have to learn about DB2. Using an SQL interface, Paradox
could access the data you want on the mainframe. Of course, Paradox must
translate your queries into SQL and forward them to the DB2 for processing. You
would have to know the names of the fields and the relations in the DB2
database.
The use of SQL as an intermediary and a standard in accessing a large number of
different types of database systems should be of great help to users and to
systems analyst. Although you may never formulate a query in SQL, you are likely
to find it processing queries developed in other languages.
