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Oracle Tips by Burleson 

Weak Entities

Weak entities have no physical value.  They are used to solve the problem created by a many-to-many link.  Since each relationship must have a unique identifier, the many-to-many link must be broken up to one-to-many links.   

The weak entityis used for this purpose.  It uses one of the primary keys as an attribute.  For example, an individual can have many hobbies; many individuals can have a particular hobby.  Thus, the link shown in figure 1.12 is many-to-many.  By adding a weak entity and relationship, I convert the link to a one-to-one between an individual and a list of hobbies, and a one-to-many between the list of hobbies and a hobby. 

So, an individual can have only one list of hobbies.  The list of hobbies can contain one or more hobbies.  Weak entities can contain attributes and will become a table in the database once the design is completed.


The above book excerpt is from:

Easy Oracle SQL

Get Started Fast writing SQL Reports with SQL*Plus

ISBN 0-9727513-7-8

John Garmany 

http://www.rampant-books.com/book_2005_1_easy_sql.htm

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