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

  • “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

  • “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

  • Mandatory Relationships

  • Optional Relationships

  • Many-to-Many Relationships

  • One-to-Many Relationships

  • One-to-One Relationships

  • Recursive Relationships


INSTRUCTOR

STUDENT

INSTRUCTOR

STUDENT

Mandatory, Many-to-Many


DEPARTMENT

STUDENT

DEPARTMENT

STUDENT

Optional, Many-to-Many


INSTRUCTOR

SKILL

INSTRUCTOR

SKILL

Optional/Mandatory,Many-to-Many


PRODUCT

VENDOR

PRODUCT

VENDOR

Optional/Mandatory,One-to-Many


AUTOMOBILE

ENGINE

AUTOMOBILE

ENGINE

Mandatory, One-to-One


Recursive

EMPLOYEE

supervises

is supervised by


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.


SALES ORDERS

INV. ITEMS

SALES ORDERS

ORDER ITEMS

INV. ITEMS

Resolving Many-to-Many Relationships


CUSTOMERS

CLERKS

SALES ORDERS

ORDER ITEMS

INV. ITEMS

Example (ER Diagram)


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