IS 325

Notes for

Wednesday

September 18, 2013

Homework Grades/Feedback• I’m behind on homework.. I will catch-up this weekend

Today’s Class• Periodically you might need to be reminded why you are here

Why It's Important to Know Computer Programming

The World is Flat

MAPPING UNARY RELATIONSHIPS• Mapping unary relationships

Unary relationships in ER diagrams are mapped in the same way as binary relationships

MAPPING UNARY RELATIONSHIPS• Mapping 1:M unary relationships

The relation mapped from an entity involved in a 1:M unary relationship contains a foreign key that corresponds to its own primary key

MAPPING UNARY RELATIONSHIPS

Mapping a 1:M unary relationship

Sample data records for the mapped relation

Client can be referred by only one client but can refer multiple clients

MAPPING UNARY RELATIONSHIPS• Mapping M:N unary relationships

In addition to the relation representing the entity involved in a unary M:N relationship, another relation is created to represent the M:N relationship itself

This new relation has two foreign keys, both of them corresponding to the primary key of the relation representing the entity involved in the unary M:N relationship

Each of the foreign keys is used as a part of the composite primary key of the new relation

MAPPING UNARY RELATIONSHIPSMapping a M:N unary relationship

Sample data records for the mapped relations

MAPPING UNARY RELATIONSHIPS• Mapping 1:1 unary relationships

Mapped in the same way as 1:M unary relationships

MAPPING UNARY RELATIONSHIPSMapping a 1:1 unary relationship

Sample data records for the mapped relation

MAPPING MULTIPLE RELATIONSHIPS BETWEEN THE SAME ENTITIES

• Mapping multiple relationships between the same entities

Each relationship is mapped

MAPPING MULTIPLE RELATIONSHIPS BETWEEN THE SAME ENTITIES

Mapping multiple relationships between the same entities

Sample data records for the mapped relations

MAPPING WEAK ENTITIES• Mapping weak entities

Weak entities are mapped in a same way as regular entities with one addition:

The resulting relation has a composite primary key that is composed of the partial identifier and the foreign key corresponding to the primary key of the owner entity

MAPPING WEAK ENTITIESMapping a weak entity

Sample data records for the mapped relations

MAPPING WEAK ENTITIESMapping a weak entity with two owners

Sample data records for the mapped relations

MAPPING WEAK ENTITIESMapping a weak entity with no partial identifier

Sample data records for the mapped relations

Example ER diagram : HAFH Realty Company Property Management Database

Example mapped relational schema: HAFH Realty Company Property Management Database

Example: Sample data records for the HAFH Realty Company Property Management Database (part 1)

Example: Sample data records for the HAFH Realty Company Property Management Database (part 2)

RELATIONAL DATABASE CONSTRAINTS

• Relational database constraints - rules that a relational database has to satisfy in order to be valid

Implicit constraints

The implicit relational database model rules that a relational database must satisfy in order to be valid

User-defined constraints

Database constraints that are added by the database designer

RELATIONAL DATABASE CONSTRAINTS

• Implicit constraints

Each relation in a relational schema must have a different name

Each relation must satisfy the following conditions:

Each column must have a different name

Each row must be unique

In each row, each value in each column must be single valued

Domain constraint - all values in each column must be from the same predefined domain

The order of columns is irrelevant

The order of rows is irrelevant

RELATIONAL DATABASE CONSTRAINTS

• More Implicit constraints

Primary key constraint - each relation must have a primary key, which is a column (or a set of columns) whose value is unique for each row

Entity integrity constraint

Referential integrity constraint

RELATIONAL DATABASE CONSTRAINTS

• User-defined constraints

Added by the database designers

RELATIONAL DATABASE CONSTRAINTS

Specific minimum and maximum cardinalities

Sample data records for the mapped relations

RELATIONAL DATABASE CONSTRAINTS

• Business rules

User defined constraints that specify restrictions on databases that are not a part of the standard notation for creating ER diagrams

RELATIONAL DATABASE CONSTRAINTS

Business rule for salary amounts

Sample data records for the mapped relation

RELATIONAL DATABASE CONSTRAINTS

Business rule for the dates of enrollment and graduation

Sample data records for the mapped relation

RELATIONAL DATABASE CONSTRAINTS

Business rule for gender of students in an organization

Sample data records for the mapped relation

MAPPING ASSOCIATIVE ENTITIES• Mapping associative entities

Associative entities are mapped into relational database constructs in the identical way as M:N relationships

Example: An M:N relationship and associative entity mapped into a relation in the same way

MAPPING TERNARY RELATIONSHIPS

• Mapping ternary relationships

Ternary relationships are used as many-to-many-to-many relationships

A new relation is created with foreign keys from the participating entities forming a composite primary key of the new relation

Example: Mapping a ternary relationship

Example: Sample data records for the mapped relations

DESIGNER-CREATED PRIMARY KEYS AND THE AUTONUMBER OPTION

• Autonumber data type option - enables automatic generation of consecutive numeric data values in a column

• Designer-created primary key - primary key column, not called for by the original requirements, added to a table by the database designer

Often used in conjunction with the autonumber data type option

DESIGNER-CREATED PRIMARY KEYS AND THE AUTONUMBER OPTION

Entity and the resulting relation

Sample data records for the relation with a designer-created primary key

Entity and the resulting relation with a designer-created primary key column

ER AND RELATIONAL MODELING• Process of requirements collection should be accompanied by

the ER modeling and then followed by mapping the ER model into a subsequent relational schema

• Some practitioners prefer to create relational schemas straight from the requirements

In such cases, the ER modeling phase is simply omitted

ER AND RELATIONAL MODELING

• Create relational schemas straight from the requirements is not advisable for following reasons

ER modeling is more suited for visualization of the requirements

Certain concepts can be visualized graphically only in ER diagrams

Every attribute is mentioned only once in the ER diagram

An ER model is a better communication and documentation device