Entity Relationship Diagrams

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Nội dung Text: Entity Relationship Diagrams

INFM 603 – Information Technology and Organizational Context Lecture 8 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. 2
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. 3
Entity Relationship Models • Mandatory Relationships • Optional Relationships • Many-to-Many Relationships • One-to-Many Relationships • One-to-One Relationships • Recursive Relationships 4
Mandatory, Many-to-Many INSTRUCTOR STUDENT INSTRUCTOR STUDENT 5
Optional, Many-to-Many DEPARTMENT STUDENT DEPARTMENT STUDENT 6
Optional/Mandatory, Many-to-Many INSTRUCTOR SKILL INSTRUCTOR SKILL 7
Optional/Mandatory, One-to-Many PRODUCT VENDOR PRODUCT VENDOR 8
Mandatory, One-to-One AUTOMOBILE ENGINE AUTOMOBILE ENGINE 9
Recursive EMPLOYEE supervises is supervised by 10
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. 11
Resolving Many-to-Many Relationships SALES ORDERS INV. ITEMS SALES ORDERS ORDER ITEMS INV. ITEMS 12
Example (ER Diagram) CUSTOMERS CLERKS SALES ORDERS ORDER ITEMS INV. ITEMS 13