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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 |
