What is Dependency?

In the context of database design, dependencies refer to the relationships between attributes in a table. Understanding these dependencies is crucial for normalization and maintaining data integrity. There are several types of dependencies, categorized based on how one attribute depends on another.

The key types of dependencies are:

1. Functional Dependency:

A functional dependency occurs when the value of one attribute uniquely determines the value of another attribute. In other words, if you know the value of attribute A, you can uniquely determine the value of attribute B. It is denoted as A → B.

2. Partial Dependency:

Partial dependency occurs when a non-prime attribute is functionally dependent on only part of the primary key. It violates the Second Normal Form (2NF), and the remedy is to decompose the table.

3. Transitive Dependency:

Transitive dependency occurs when an attribute is functionally dependent on another non-prime attribute, which, in turn, is functionally dependent on the primary key. It violates the Third Normal Form (3NF), and the remedy is also to decompose the table.

4. Multivalued Dependency:

Multivalued dependency occurs when a non-prime attribute is dependent on a set of values (a multivalued attribute) rather than on the primary key. It is addressed in the Fourth Normal Form (4NF).

5. Composite Dependency:

Composite dependency refers to a dependency in which an attribute depends on a combination of other attributes, rather than on each individually.

4. Join Dependency:

Join dependency occurs when the values in one table can be determined by combining the values in two or more other tables. It is addressed in higher normal forms like the Fifth Normal Form (5NF).

Understanding these dependency types is crucial for designing databases that adhere to the principles of normalization, which involves organizing data efficiently to reduce redundancy and improve data integrity. Each normal form addresses specific types of dependencies to ensure that the database is well-structured and maintains consistency.