Entity Relationship Diagrams

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An entity relationship diagram is the basic design model for any database. It is the major deliverable for a conceptual database design. It is the logical organization of any data in the database and is usually made up of three components namely; entities, attributes and relationships. Entities are the major categories of data in the database and are usually represented by rectangles. Attributes are the characteristics of the entities and are usually written in the rectangle. Relationships are the connections between entities and are usually represented using lines joining the rectangles. Entities that share major characteristics are known as entity types, and are also referred to as classes. An entity instance is the individual occurrence of one entity type. It is important to define entity types as it makes it easier to determine the requirements of the database and how it will be structured.
In defining an attribute, the analyst must state the importance of the attribute, the value of the attribute and whether the value of this attribute can change. When drawing an entity relationship diagram, the analyst must be aware of the following types of attributes; multi-valued attributes, required attributes, optional attributes, composite attributes and derived attributes. When showing relationships, an analyst must be aware of three types of relationships; unary (between instances of one entity type), binary (between instances of two entity types), and ternary (between
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