Entity relationship diagrams
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Entity Relationship Diagrams. Entity. “An entity is a business object that represents a group, or category of data.” 1 Do we know a similar concept? . 1) Stephens, R.K. and Plew. R.R., 2001. Database Design . SAMS, Indianapolis , IN. Attribute.

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Entity Relationship Diagrams

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Entity relationship diagrams

Entity Relationship Diagrams


Entity

Entity

  • “An entity is a business object that represents a group, or category of data.”1

  • Do we know a similar concept?

1) Stephens, R.K. and Plew. R.R., 2001. Database Design. SAMS, Indianapolis , IN.


Attribute

Attribute

  • “An attribute is a sub-group of information within an entity.”1

  • Do we know a similar concept?

1) Stephens, R.K. and Plew. R.R., 2001. Database Design. SAMS, Indianapolis , IN.


Entity relationship models

Entity Relationship Models

  • Mandatory Relationships

  • Optional Relationships

  • Many-to-Many Relationships

  • One-to-Many Relationships

  • One-to-One Relationships

  • Recursive Relationships


Mandatory many to many

INSTRUCTOR

STUDENT

INSTRUCTOR

STUDENT

Mandatory, Many-to-Many


Optional many to many

DEPARTMENT

STUDENT

DEPARTMENT

STUDENT

Optional, Many-to-Many


Optional mandatory many to many

INSTRUCTOR

SKILL

INSTRUCTOR

SKILL

Optional/Mandatory,Many-to-Many


Optional mandatory one to many

PRODUCT

VENDOR

PRODUCT

VENDOR

Optional/Mandatory,One-to-Many


Mandatory one to one

AUTOMOBILE

ENGINE

AUTOMOBILE

ENGINE

Mandatory, One-to-One


Recursive

Recursive

EMPLOYEE

supervises

is supervised by


Resolving many to many relationships

Resolving Many-to-Many Relationships

  • Many-to-many relationships should be avoided. We can resolve a many-to-many relationship by dividing it into two one-to-many relationships.


Resolving many to many relationships1

SALES ORDERS

INV. ITEMS

SALES ORDERS

ORDER ITEMS

INV. ITEMS

Resolving Many-to-Many Relationships


Example er diagram

CUSTOMERS

CLERKS

SALES ORDERS

ORDER ITEMS

INV. ITEMS

Example (ER Diagram)


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