Transcript chap4 seq9.ppt

System Analysis and Design
System Design
- Mr. Ahmad Al-Ghoul
learning Objectives
 Explain data design terminology,
including entitles, fields, records,
files, tables, and key fields
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Data Design Terminology
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Data model
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A formal way of representing the data that are used
and created by a business system
Shows the people, places and things about which
data is captured and the relationships among them.
Logical data model
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shows the organization of data without indicating how
it is stored, created, or manipulated.
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Data Design Terminology
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Entity: An entity is a person, place, thing,
or event for which data is collected and
maintained.
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eg.
Must be multiple occurrences to be an entity
EMPLOYEE
SALES ORDER
SUPPLIER
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often identified from nouns used within the business
application
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Data Design Terminology
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Entity Type
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An Entity Type is a logical grouping of data
relevant to the application in question
An Entity instance is an Occurrence of an
Entity Type
An Entity Type would be represented in a
RDBMS as a table.
An Entity instance would be represented as a
single row in a table
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Data Design Terminology
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Table or file: A set of related records is grouped into a
table, which stores data about a specific entity. Tables
are shown as two-dimensional structures that consist of
vertical columns representing fields and horizontal rows
representing records.
Field: A single characteristic or fact about an entity. A
field, or attribute, is the smallest piece of data that has
meaning within an information system. For example, a
Social Security number or company name could be
examples of a field. The terms, data element and field,
are used interchangeably.
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Attribute names are nouns
Sometimes entity name is added at the beginning of the
attribute name for clarity
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Data Design Terminology
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Common field: A common field is an attribute
that appears in more than one entity. Common
fields can be used to link entities in various
types of relationships.
Record: also called a tuple, is a set of related
fields that describes one entity instance, or
member of an entity, such as one customer, one
order, or one product. A record might have one
or dozens of fields, depending on what
information is needed.
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Data Design Terminology
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Key Fields
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Key fields are used during the systems design
phase to organize, access, and maintain data
structures. The four types of key fields are
primary keys, candidate keys, foreign keys,
and secondary keys.
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Data Design Terminology
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Key Fields
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Primary key: A primary key is a field or combination of fields that
uniquely and minimally identifies a particular member of an
entity. An identifier may be ‘artificial,’ such as creating an ID
number. For example, in a student table the student number is
a unique primary key because no two students can have the
same student number. That key also is minimal because it
contains no information beyond what is needed to identify the
student. Sometimes it is necessary for a primary key to consist
of a combination of fields. In that case, the primary key is called
a combination key, composite key, concatenated key, or multivalued key. For example, if a student registers for three courses,
his or her student number will appear in three records so its no
more valid as a primary key. So in the registration file, neither
the student number nor the course ID is unique, to identify a
specific student in a specific course, the primary key must be a
combination of student number and course ID
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No two fields within the primary key can have the same value
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Data Design Terminology
Attribute Names
Primary
Key
Columns
Field
R
e
c
o
r
d
Supplier
Number
Supplier Name
Supplier Address
Supplier
Tel. No.
Sup
Con
1463
Ahmad
115 zeriab St
+962/7563025
Ali
3621
Ali & sons
12 King Hussien
St
020-8…
...
2327
Bella Sonic
Lake Industrial
Estate
+962/7999663
...
6762
…
3 Lot’s Corner
...
...
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Data Design Terminology
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Key Fields
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Candidate key: Sometimes it is possible to have a
choice of fields or field combinations to use as the
primary key. Any field that could serve as a primary
key is called a candidate key. For example, if every
employee has a unique employee number, then you
could use either the employee number or the Social
Security number as a primary key.
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Nonkey field: Any field that is not a primary key or a
candidate key is called a nonkey field.
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Data Design Terminology
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Key Fields
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Foreign key: A foreign key is a field in one table that
must match a primary key value in another table in
order to establish the relationship between the two
tables. Unlike a primary key, a foreign key need not
be unique
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A foreign key must either match a primary key or else be null
Secondary key: is a field or combination of fields that
can be used to access or retrieve records. Secondary
key values are not unique. For example, if you need
to access records for only those customers in a
specific ZIP code, you would use the ZIP code field as
a secondary key. Secondary keys also can be used to
sort or display records in a certain order.
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Data Design Terminology
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Entity Integrity
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Entity integrity constraints are rules for
primary keys:
The primary key cannot have a null value
 If the primary key is a composite key, none of the
fields in the key can contain a null value
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Data Design Terminology
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Referential Integrity
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Validity checks can help avoid data input errors
One type of validity check, called referential integrity, is
a set of rules that a voids data inconsistency and
quality problems
In a relational database, referential integrity means
that a foreign key value cannot be entered in one table
unless it matches an existing primary key in another
table
You cannot change a primary key that has matching
child table records
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A child table that has a foreign key for a different record
Referential integrity also can prevent the deletion of a
record has a primary key that matches foreign keys in
anther table
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Data Design Terminology
Secondary key
Foreign key
Primary keys
Secondary key
Candidate keys
Combination
primary key
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Sequence Summary
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In an information system, an entity is a person, place, thing, or
event for which data is collected and maintained
A field, or attribute, is a single characteristic of an entity
A record, or tuple, is a set of related fields that describes one
instance of an entity
Records are grouped into files (in a file-oriented system) and
tables (in a database environment)
A primary key is a field or field combination that uniquely and
minimally identifies a specific record; a candidate key is any field
that could serve as a primary key
A foreign key is a field or field combination that must match
primary key of anther file or table
A secondary key is a field or field combination used as the basis
for sorting or retrieving records
Entity integrity constraints are rules for primary keys
referential integrity, is a set of rules that a voids data
inconsistency and quality problems
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Sequence Summary
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In this Sequence we have
 Explain data design terminology, including
entitles, fields, records, files, tables, and key
fields
 Focused on entity integrity and referential
integrity
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Reference
[1] System Analysis and Design, Sixth
Edition
Authors: Gary B. Shelly, Thomas J.
Cashman and Harry J. Rosenblatt
Publisher: SHELLY CASHMAN SEWIES.
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