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Passage Two.
Relational Database and Structured Query Language
Training target:
In this part，our target is to train your reading
speed. You should pay more attention to” word group”.
Treating a group of words as a whole unit is a short cut
to save your time. We have underlined the word groups
in some sentences，and the rest can be practiced by
yourself. If there are some new words，you may cover
the note area with a piece of paper and try to guess
their meanings without looking them up in the
dictionary.
 Text
 1.Relational Database
 A database is similar to a data file in that it is a storage place for data.
Like a data file，a database does not present information directly to a
user，the user runs an application that accesses data from the
database and presents it to the user in an understandable format.
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 Although there are different ways to organize data in a database，
relational databases are one of the most effective. Relational
database systems are an application of mathematical set theory to
the problem of effectively organizing data. In a relational database，
data is collected into tables（called relations in relational theory）.
.
A table represents some class of objects that are important to an
organization. For example，a company may have a database with a
table for employees，another table for customers，and another for
stores. Each table is built of columns and rows. Each column
represents some attribute of the object represented by the table. For
example，an Employee table would typically have columns for
attributes such as first name，last name，employee ID，department，
pay grade，and job title. Each row represents an instance of the
object represented by the table. For example，one row in the
Employee table represents the employee who has employee ID
12345.
 .When organizing data into tables，you can usually find many
different ways to define tables. Relational database theory defines a
process called normalization，which ensures that the set of tables
you define will organize your data effectively.
2.Structured Query Language normalization
 To work with data in a database，you have to use a set of commands
and statements（language）defined by the DBMS software. Several
different languages can be used with relational databases，the most
common is SQL.SQL is an abbreviation of structured query language，
and pronounced either See Kwell or as separate letters. SQL is a
standardized query language for requesting information from a
database. The original version called SEQUEL（structured English
query language）was designed by an IBM research center in 1974 and
1975.
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 SQL was first introduced as a commercial database system in1979by
Oracle Corporation. The American National Standards Institute
（ANSI）and the International Standards Organization（ISO）define
software standards，including standards for the SQL language. SQL
Server2000supports the Entry Level of SQLˉ92，the SQL standard
published by ANSI and ISO in1992.The dialect of SQL supported by
Microsoft SQL Server is called TransactˉSQL（TˉSQL）.TˉSQL is the
primary language used by Microsoft SQL Server applications.
.
 Historically，SQL has been the favorite query language for database
management systems running on minicomputers and mainframes.
Increasingly，however，SQL is being supported by PC database
systems because it supports distributed databases（databases that
are spread out over several computer systems）.This enables
several users on a local area network to access the same database
simultaneously.
 Although there are different dialects of SQL，it is nevertheless the
closest thing to a standard query language that currently exists.
In1986，ANSI approved a rudimentary version of SQL as the official
standard，but most versions of SQL since then have included many
extensions to the ANSI standard.In1991，ANSI updated the standard.
The new standard is known as SAG SQL（SQL Access Group）.
 SQL contains about60commands and is used to create，modify，
query，and access data organized in tables. It can be used either as
an interactive interface or as embedded commands in an application:
 （1）Dynamic SQL statements are interactive，and they can be
changed as needed. If you normally access SQL from a commandline environment，you are using dynamic SQL，which is slower than
static SQL but much more flexible.
 （2）Static SQL statements are coded into application programs，
and as a result，they do not change. This statements are usually
processed by a precompiler before being bound into the application.

SQL is such a popular standard today，every major client/server
application supports it. Many database implement SQL queries
behind the scenes，enabling communication with database servers
in systems with client/server architecture.
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The End