Entity Relationship Diagrams

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Entity Relationship Diagrams INFM 603 – Information Technology and Organizational Context Lecture 8Entity 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-ManyINSTRUCTOR STUDENTINSTRUCTOR STUDENT 5 Optional, Many-to-ManyDEPARTMENT STUDENTDEPARTMENT STUDENT 6 Optional/Mandatory, Many-to-ManyINSTRUCTOR SKILLINSTRUCTOR SKILL 7 Optional/Mandatory, One-to-ManyPRODUCT VENDORPRODUCT VENDOR 8 Mandatory, One-to-OneAUTOMOBILE ENGINEAUTOMOBILE ENGINE 9 Recursive EMPLOYEE supervisesis 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. ITEMSSALES ORDERS ORDER ITEMS INV. ITEMS 12 Example (ER Diagram)CUSTOMERS CLERKS SALES ORDERS ORDER ITEMS INV. ITEMS 13