Dependency Diagrams

Dependency diagrams#

A dependency diagram illustrates the various dependencies that might exist in a non-normalized table. A non-normalized table is one that has data redundancy in it. This is illustrated below:

As we can observe from the table above, Project_No and Emp_No, combined, are the primary key (as the combination of these two attributes can be used to identify each record uniquely).

The following dependencies can be identified from this table:

Partial dependencies:

• Project_No $\rightarrow$ Proj_Name

Reason: Since the name of the project is only dependent upon part of the multi-attribute PK, i.e., Project_No, this results in partial dependency.

• Emp_No $\rightarrow$ Emp_Name, Dept_No

Reason: Similar to the example above, we see that these two attributes only depend on part of the composite key and not the whole.

Transitive dependency:

• Dept_No $\rightarrow$ Dept_Name

Reason: Since a non-key attribute (Dept_Name) is dependent on another non-key attribute (Dept_No), this results in a transitive dependency.

We have highlighted some of the dependencies that exist in this table, but there are many more.

The next lesson will include a short quiz to test your knowledge of functional dependencies.