Database Reference
In-Depth Information
language constructs provide the flow of control logic and also the presentation of
results on GUI screens or reports.
Major Aspects of SQL
•
Data definition language component
—for creation, deletion, and modification
of definitions for relational tables and user views; for creation and dropping
of indexes (as provided in commercial versions); for specifying integrity
constraints
•
Data manipulation language component
—consisting of a query language based
on relational calculus and some relational algebra
•
Data maintenance facility
—to insert, modify, or delete rows from relational
tables
•
Security control
—to grant or revoke access privileges to users
•
Transaction control
—commands specifying beginnings and endings of transac-
tions with explicit locking by some implementations for concurrency control
•
Embedded SQL
—providing calls from host languages
•
Dynamic SQL
—for constructing and executing queries dynamically at run time
•
Triggers
—actions coded as program modules executed by DBMS whenever
changes to the database meet conditions stipulated in the trigger
•
Client/server adaptation
—for client application programs to connect to an SQL
database server
•
Remote access
—access of relational database over a network
Brief History and Evolution
It all began in the early 1970s when Dr. E. F. Codd of IBM's San Jose Research
Laboratory proposed the relational data model. IBM developed a prototype DBMS
called System R to validate the feasibility of the relational data model. D.
Chamberlin, from the same laboratory, defined a language called the Structured
English Query Language as the language interface for System R. This language
came to be known as SEQUEL. However, SEQUEL owes its beginning to another
language called SQUARE (Specifying Queries as Relational Expressions), designed
to implement relational algebra with English, which formed the basis for the devel-
opment of SEQUEL. Development of SEQUEL, later known as SQL (pronounced
as
es-que-el
), formed the high point of the relational model projects at San Jose.
In the late 1970s, Oracle became the first commercial implementation of a rela-
tional DBMS based on SQL. INGRES followed shortly thereafter as an imple-
mentation of relational DBMS with QUEL as the interface language. IBM's first
commercial DBMS on the relational data model known as SQL/DS appeared in
1981 for DOS/VSE environments, and in 1982 for the VM/CMS environments. Sub-
sequently, in 1983 IBM introduced DB2 for MVS environments. Now the relational
DBMS of every vendor is based on SQL. Each vendor provides all the standard
features of SQL. Each vendor also provides additional features of its own, called
extensions to standard SQL. Vendors attempt to distinguish their SQL versions with
these extensions.
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