Done By : Gillian Swaby

Objectives: 1. What is an Entity-Relationship Diagram? 2.What are the symbols used in ERD? Explain each. 3. What is a one- to- one relationship? Give examples. 4. What is a many- to- one relationship? Give examples. 5. What is a many- to- many relationship? Give examples.

Entity- Relationship Diagram An entity- relationship diagram is a specialized graphic that illustrates the inter-relationships between entities in a database. Is a visual representation of all data stored in a system and their relationships.

An example of an ER-diagram movie directortitle

Symbols used in an ER-diagram Boxes- these are commonly used to represent entities Diamonds- are normally used to represent relationships. Ovals- are used to represent attributes.

One – to – one relationship One- to- one relationship occur when there is exactly one record in the 1 st table that corresponds to exactly one record in the related table.

One- to- many relationship One – to- many relationships occur when each record in Table A may have many linked records in Table B but each record in Table B may have only one corresponding record in Table A.

Many- to- many Relationship (M:M) is used to relate many records in Table A with many records in Table B