E R Diagram
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An entity relationship diagram models the relationships between entities in a database. It shows entities, attributes, and different types of relationships between entities including mandatory, optional, one-to-one, one-to-many, many-to-many, and recursive relationships. Many-to-many relationships should be avoided and can be resolved by introducing a joining entity to split them into two one-to-many relationships.
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E R Diagram
- 2. 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.
- 3. 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.
- 4. Entity Relationship Models Mandatory Relationships Optional Relationships Many-to-Many Relationships One-to-Many Relationships One-to-One Relationships Recursive Relationships
- 5. Mandatory, Many-to-Many INSTRUCTOR STUDENT INSTRUCTOR STUDENT
- 6. Optional, Many-to-Many DEPARTMENT STUDENT DEPARTMENT STUDENT
- 7. Optional/Mandatory, Many-to-Many INSTRUCTOR SKILL INSTRUCTOR SKILL
- 8. Optional/Mandatory, One-to-Many PRODUCT VENDOR PRODUCT VENDOR
- 9. Mandatory, One-to-One AUTOMOBILE ENGINE AUTOMOBILE ENGINE
- 10. Recursive EMPLOYEE supervises is supervised by
- 11. 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.
- 12. Resolving Many-to-Many Relationships SALES ORDERS INV. ITEMS SALES ORDERS INV. ITEMS ORDER ITEMS
- 13. Example (ER Diagram) SALES ORDERS INV. ITEMS ORDER ITEMS CLERKS CUSTOMERS