Terabytesized databases Aterabyteis aSI-multiple (see prefixtera-) of the unitbytefor digital informationstorageand is equal to 1012(1000000000000bytes) or 1000gigabytes. The terabyte is abbreviated with the
symbolTB.
Definition of table
in database: table:
A single store of related information. A table consists of records, and
each record is made up of a number of fields. Just to totally confuse
things, tables are sometimes called relations. You can think of the
phone book as a table: It contains a record for each telephone
subscriber, and each subscriber's details are contained in three fields
- name, address and telephone.
Fields &
attributes:
Once you have decided on your initial attributes
(database fields), you may need to look into how to break this
attribute down, a process more commonly known as decomposing
attributes. Decomposing an attribute takes the attribute from its
original form and divides it into its own components. A simple example
of this might be the breaking down of an Address attribute. An Address
might easily be broken down and defined using the following attributes
to store all of its data:
Street - stores the Street address for the record
Town - stores the Town address for the record
County - stores the County information for the record
PostCode - stores the PostCode information of the record
Entity
or Relationship?
An easy way to decide whether an object should be an entity or a
relationship is to map nouns in the requirements to entities, and to
map the verbs to relations. For example, in the statement, “A degree
program is made up of one or more courses,” we can identify the
entities “program” and “course,” and the relationship “is made up of.”
Similarly, in the statement, “A student enrolls in one program,” we can
identify the entities “student” and “program,” and the relationship
“enrolls in.” Of course, we can choose different terms for entities and
relationships than those that appear in the relationships, but it’s a
good idea not to deviate too far from the naming conventions used in
the requirements so that the design can be checked against the
requirements. All else being equal, try to keep the design simple, and
avoid introducing trivial entities where possible; i.e., there’s no
need to have a separate entity for the student’s enrollment when we can
model it as a relationship between the existing student and program
entities.
The ER diagram representation of student
grades as attributes of the student entity
The ER diagram representation of student
grades as a separate entity
Database schema:
The schema of a database system is its structure formal language
supported by the database management system (DBMS). In a relational
database, the schema defines the tables, the fields in each table, and
the relationships between fields and tables. Schemas are generally
stored in a data dictionary. Although a schema is defined in text
database language, the term is often used to refer to a graphical
depiction of the database structure. Example:
Online Database Schema Samples Library
SQL (Structured Query
Language) is a database computer language designed for the
retrieval and management of data in relational database management
systems (RDBMS), database schema creation and modification, and
database object access control management. SQL
is a querying language for
querying and modifying data and managing databases. SQL
was standardized first by
the ANSI and later by the ISO. Most database management systems
implement a majority of one of these standards and add their
proprietary extensions. SQL
allows the retrieval, insertion, updating, and deletion of data. A
database management system also includes management and administrative
functions. Most – if not all – implementations also include a
command-line interface (SQL/CLI) that allows for the entry and
execution of the language commands, as opposed to only providing an
application programming interface (API)
intended for access from a graphical user interface (GUI).
A Database
Management System (DBMS) is a set of software programs that controls the organization, storage, management, and retrieval of data in a database.
DBMS are categorized according to their data structures or types. It is
a set of prewritten programs that are used to store, update and
retrieve a Database. The DBMS accepts requests for data from the
application program and instructs the operating system to transfer the appropriate data. When a
DBMS is used, information systems
can be changed much more easily as the organization's information
requirements change. New categories of data can be added to the
database without disruption to the existing system.
Often abbreviated DB.
A collection of information organized in such a way that a computerprogram can quickly select desired pieces of data. You can think of a database as an
electronic filing system.
Traditional databases are organized by fields,
records,
and files.
A field is a single piece of information; a record is one complete set
of fields; and a file is a collection of records. For example, a
telephone book is analogous to a file. It contains a list of records,
each of which consists of three fields: name, address, and telephone
number.
An alternative concept in database
design is known as Hypertext.
In a Hypertext database, any object, whether
it be a piece of text,
a picture, or a film, can be linked
to any other object. Hypertext databases are particularly useful for
organizing large amounts of disparate information, but they are not
designed for numerical analysis.
To access
information from a database, you need a database
management system (DBMS). This is a collection of programs that
enables you to enter, organize, and select data in a database.
Primary Key
In relational database design, a unique
key or primary key is a candidate
key to uniquely identify each row in a table. A unique key or primary key
comprises a single column
or set of columns. No two distinct rows in a table can have the same
value (or combination of values) in those columns. Depending on its
design, a table may have arbitrarily many unique keys but at most one primary
key.
A unique key must uniquely identify all possible
rows that exist in a table and not only the currently existing rows.
Examples of unique keys are Social Security numbers (associated
with a specific person)
or ISBNs (associated with
a specific book). Telephone books and dictionaries
cannot use names or words or Dewey Decimal system numbers as candidate keys
because they do not uniquely identify telephone numbers or words.
A primary key is a special case of
unique keys. The major difference is that for unique keys the implicit NOT
NULL
constraint is not automatically enforced, while for primary keys it is.
Thus, the values in a unique key columns may or may not be NULL.
Another difference is that primary keys must be defined using another
syntax.
The relational model, as expressed
through relational calculus and
relational algebra, does not distinguish between primary keys and other
kinds of keys. Primary keys were added to the SQL standard
mainly as a convenience to the application programmer.
boolean
data-type
In computer science, the Boolean datatype, sometimes
called the logical datatype, is a primitive datatype
having one of two values: true
and false. Many systems represent true as non-zero
(often 1, or -1) and false as zero. It is the special case of a
binary numeric datatype of only one
digit, or bit,
and can also be represented in any other radix by
restricting the range of allowed values for certain operations.
In some languages the Boolean datatype
is defined to represent more
than two truth values. For instance the ISO SQL:1999 standard defined a
Boolean data type for SQL which could hold three possible values: true,
false, unknown (SQL null
is treated as equivalent to the unknown truth value, but only for the
Boolean data type). This defies the law of excluded middle, though very
often useful.
In semiconductor
logic hardware, it is usually more efficient to implement all as NAND or
NOR, that is NOT-AND and NOT-OR (which is not the same as NOR).
Datasheet
view In the Button Bar, just
to the right of Help (in the Menu
Bar), is a button with a
lightening bolt and a small form.
This is the New Object: AutoForm
Button. Point to it, make sure you have the correct button,
then click-on it. A New
Data Entry Form will automatically be created and appear.
"COUNT( )" is a SQL function used to
count the number of rows in a query.
"SELECT
COUNT (*) FROM Sales" is a statement
that select the total number of orders from the Sales table.
"DELETE"
is a SQL clauses used to DELETE data from a database table.
When
inserting data in a table, you don't
have
to specify a list of all column names you are inserting values for.
"ORDER
BY" is a SQL
clauses used to sort a result set.
"SELECT"
is a SQL
commands used to retreive data.
SELECT * FROM TABLE_1 WHERE UPPER(COLUMN_2)
= 'GOMEZ'; Here we're using an expression on the
column, an operation which would be
necessary if you wanted to find 'Gomez' as well as 'GOMEZ'. For a full
description of the various operators and functions that you can use in
expressions, see
Functions and
Operators .
This is a correct SQL syntax "SELECT
* FROM Table 1 WHERE Column 1 >= 10"
It is true that Databases
and database technology are having a major impact on the growing use of
computers.
The
query compiler handles high-level queries that are entered
interactively. Parses,
analyses and interprets
are some important actions of the query compiler.
