Before you can create databases and other database objects, it's important that you understand basic relational concepts. Without such understanding, you will not be able to implement your databases effectively.
This module outlines some basic principals of relational databases.
The ability to make changes to the logical layer without disrupting existing users and processes is called logical data independence
It is the transformation between the logical layer and the external layer that provides logical data independence. As with physical data independence, there are degrees of logical data independence. It is important that you understand that most logical changes also involve a physical change
For example, you cannot add a new database object (such as a table in a relational DBMS) without physically storing the data somewhere; hence, a corresponding change is made in the physical layer. Moreover, deletion of objects in the logical layer will cause anything that uses those objects to fail but should not affect anything else. Here are some examples of changes in the logical layer
that can be safely made thanks to logical data independence:
- Adding a new database object
- Adding data items to an existing object
- Making any change in which a view can be placed in the external model that replaces (and processes the same as) the original object in the logical layer, such as combining or splitting existing objects
In the next lesson, you will learn all about the concepts underlying relational databases.