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by Burleson Consulting

The Data Warehouse Development Life Cycle

Oracle Data Warehouse Design
De-normalizing Many-To-Many Data Relationships

In many cases, a many-to-many relationship can be condensed into a more efficient structure to improve the speed of data retrieval. After all, less tables will need to be joined to get the desired information. Using the relationship between a course and a student as an example, Figure 4.7 shows how a many-to-many relationship can be collapsed into a more compact structure.

Figure 4.7  Collapsing a many-to-many relationship into a more compact structure.

A student takes many courses, and each course has many students. This is a classical many-to-many relationship and requires us to define a junction table between the base entities to establish the foreign keys necessary to join them together. Note that the GRADE  is the junction table, and it contains the following contents: course_nbr, the primary key for the COURSE table; student_nbr, the primary key for the STUDENT table; and grade, which is a no-key attribute for both foreign keys. Next, consider the following: In what context does a grade have meaning? Simply stating that The grade is A in CS-101 is insufficient, and stating Joe earned an A makes no sense, either. Only when both the student number and the course number are associated does the grade column have meaning. Stating Joe earned an A in CS-101 makes sense.

This is an excerpt from "High Performance Data Warehousing". To learn more about Oracle, try "Oracle Tuning: The Definitive Reference", by Donald K. Burleson.  You can buy it direct from the publisher at 30% off here:


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