Datamining And Security

SQL вЂ" Structured Query Language

1.1 Tables

In relational database systems (DBS) data are represented using tables (relations ). A query

issued against the DBS also results in a table. A table has the following structure:

Column 1 Column 2 . . .

Column n

в†Ñ'в?' Tuple (or Record)

. . .

. . .

. . .

. . .

A table is uniquely identiп¬Ðƒed by its name and consists of rows that contain the stored informa-

tion, each row containing exactly one tuple (or record ). A table can have one or more columns.

A column is made up of a column name and a data type, and it describes an attribute of the

tuples. The structure of a table, also called relation schema, thus is deп¬Ðƒned by its attributes.

The type of information to be stored in a table is deп¬Ðƒned by the data types of the attributes

at table creation time.

SQL uses the terms table, row, and column for relation, tuple, and attribute, respectively. In

this tutorial we will use the terms interchangeably.

A table can have up to 254 columns which may have diп¬Ð‚erent or same data types and sets of

values (domains), respectively. Possible domains are alphanumeric data (strings), numbers and

date formats. Oracle oп¬Ð‚ers the following basic data types:

• char(n): Fixed-length character data (string), n characters long. The maximum size for

n is 255 bytes (2000 in Oracle8). Note that a string of type char is always padded on

right with blanks to full length of n. ( can be memory consuming).

Example: char(40)

• varchar2(n): Variable-length character string. The maximum size for n is 2000 (4000 in

Oracle8). Only the bytes used for a string require storage. Example: varchar2(80)

• number(o, d): Numeric data type for integers and reals. o = overall number of digits, d

= number of digits to the right of the decimal point.

Maximum values: o =38, d= в?'84 to +127. Examples: number(8), number(5,2)

Note that, e.g., number(5,2) cannot contain anything larger than 999.99 without result-

ing in an error. Data types derived from number are int[eger], dec[imal], smallint

and real.

• date: Date data type for storing date and time.

The default format for a date is: DD-MMM-YY. Examples: ’13-OCT-94’, ’07-JAN-98’

1

• long: Character data up to a length of 2GB. Only one long column is allowed per table.

Note: In Oracle-SQL there is no data type boolean. It can, however, be simulated by using

either char(1) or number(1).

As long as no constraint restricts the possible values of an attribute, it may have the special

value null (for unknown). This value is diп¬Ð‚erent from the number 0, and it is also diп¬Ð‚erent

from the empty string вЂ™Ð²Ð‚™.

Further properties of tables are:

• the order in which tuples appear in a table is not relevant (unless a query requires an

explicit sorting).

• a table has no duplicate tuples (depending on the query, however, duplicate tuples can

appear in the query result).

A database schema is a set of relation schemas. The extension of a database schema at database

run-time is called a database instance or database, for short.

1.1.1 Example Database

In the following discussions and examples we use an example database to manage information

about employees, departments and salary scales. The corresponding tables can be created

under the UNIX shell using the command demobld. The tables can be dropped by issuing

the command demodrop under the UNIX shell.

The table EMP is used to store information about employees:

EMPNO ENAME JOB

7369 SMITH CLERK

MGR HIREDATE SAL DEPTNO

7902 17-DEC-80 800 20

7499 ALLEN SALESMAN 7698 20-FEB-81 1600 30

7521 WARD SALESMAN 7698 22-FEB-81 1250 30

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