Introduction to Oracle9i - SQL - Student Guide Vol 2

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As stated, a many-to-many relationship is implemented using three entities. The two main entities are connected together using a third entity. The third entity contains keys connected to the other two entities. In our basic data model the Order Detail entity is just such an entity. In the Northwind example this entity, however, also exists in its own right and contains additional attributes of its own. In many models the third entity is created for the purpose of joining two other entities and has no other attributes except for those needed as key values to connect to the original entities.

They will become the columns or fields in the physical database. • Relationships show a logical link between two or more entities. Where two entities have a direct affiliation, a relationship is used to define the connection between the entities. A supplier entity may be related to orders; a product entity may be related to purchases. Relationships will normally establish constraints within the physical database. Entities represent the primary elements of the ER Model. Each entity will represent a person, place, thing, or concept involved in a business process.

A rough draft of Northwind's entities. With further knowledge of the business process, other entities may come to light. In the case of the Northwind process, products all fall into separate categories and the product categories are also tracked. 2. 2. Northwind's entities in an ER Model. [View full size image] 40 41 Entity structuring accommodates the initial stages of database design. When you're designing an appropriate logical model, the data must be organized into these separate elements that will later make up the physical database tables.

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Introduction to Oracle9i - SQL - Student Guide Vol 2
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