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Oracle Tips by Burleson |
Inside Data Relationships
As we know, the data relationships associate the
entities with each other and allow us to join them together.
Initially in the ERD XE "ERD" , relationships are defined with words.
In Figure 1.10, the employee entity works in a
department entity. Links are defined as to the cardinality of the
entities in the relationship. For example, an employee can only work
in zero or one department. The link between the employee and the
department is a one-to-one (or one-to-none).
A department can have many employees, so the
relationship between departments and employees is one-to-many (one
dept, many employees). If an employee could be in more than one
department, then the link would be many-to-many. Each tool uses a
method of defining the cardinality of a link. Here are some examples
in Figure 1.11.
The above book excerpt is from:
Easy Oracle
SQL
Get Started
Fast writing SQL Reports with SQL*Plus
ISBN 0-9727513-7-8
Col. John Garmany
http://www.rampant-books.com/book_2005_1_easy_sql.htm |
