Partial Dependency in DBMS

Databases are structured to handle cases where some data relies on only part of a key, not the whole key. In this case we can say data is partially dependent. Partial dependency is similar to a functional or strong dependency, as it shows a constrained relationship between two or more attributes in a table.

Key terms to know before we learn more about partial dependency:

• Database: Organized body of information that is stored electronically, such as an electronic filing system.
• Functional Dependency (X → Y): A dependency in which attribute X determines attribute Y uniquely.
• Attribute: A characteristic or attribute of an entity (e.g., StudentName in a Student table).
• Prime Attribute: An attribute(column) that is included in any candidate/primary key.
• Non-Prime Attribute: An attribute(column) not included in any candidate/primary key.
• Relation (Table): A database organization holding data in rows (records) and columns (attributes).
• Candidate Key: A minimum number of attributes that uniquely identify a record (several possible per table).
• Primary Key: The selected candidate key that uniquely identifies every record (just one per table).

What is Partial Dependency?

In a database, a partial dependency occurs when a non-key column (i.e., not part of any candidate key) depends on only part of a composite primary key instead of the full key. This is explained in detail with the help of below given example.

Example

Explanation:

• Candidate Key: {Student_ID, Course_ID}
• Non-Prime Attribute: Course_Name, Instructor
• Partial Dependency: Course_Name → Course_ID (since Course_Name depends on part of the primary key, which is Course_ID)

To resolve this dependency we will use normalization. For normalizing this table we will split it into two tables:

1. Student_Course:

2. Course:

How is Partial Dependency Identified?

Partial dependency can be identified by testing the functional dependencies between the attributes in a table. Functional dependencies describe how one or more attributes rely on another attribute, which may be within the same table or across different tables. In the case of partial dependency, an attribute depends only on part of the primary key.

If Course_Name is only dependent on Course_ID (the primary key), then Course_Name is partially dependent on the primary key.

One way to detect partial dependency is by scanning the table for similar or repeated data. This can be tricky when the data isn’t clearly organized. Repeated data can lead to inconsistencies and discrepancies, which might not be obvious without examining the table for patterns of data duplication.

How to Minimize Partial Dependency?

Normalization of tables holds the key to eliminating potential dependencies among attribute elements. Normalization is a process aimed at refining a database to reduce redundancy and ensure data consistency.

There are different levels of normalization, each with its own set of rules and requirements:

• The first level of normalization, known as First Normal Form (1NF), requires that all attributes in a table have atomic values. This means each attribute should contain only a single value and cannot contain multiple values. By doing so, we eliminate repetitive data and allow for partial independence.

• The next level is Second Normal Form (2NF), which requires that every non-key attribute be fully dependent on the primary key. This ensures that attributes that are only partially dependent on the primary key are moved to a separate table. This separation enables the database to maintain consistency and operate independently.

• The third level is Third Normal Form (3NF), which stipulates that every non-key attribute must be independent of other non-key attributes. In other words, if an attribute is related to another non-key attribute in the same table, it must be moved to a different table. This eliminates transitive dependencies and ensures that the data is more fully normalized.

Beyond the third normal form, there are higher levels of normalization, such as Fourth Normal Form (4NF) and Fifth Normal Form (5NF). However, these forms are rarely used in practice.

Example to Minimize Partial Dependency

Original Table (CourseEnrollment):

Here the department is partially dependent on the instructor. While an instructor typically teaches courses in his or her own department, an instructor may also teach courses from other departments. This creates a partial dependency. We will use normalization to reduce partial dependency.

Decompose the table into two separate tables:

• CourseEnrollment (StudentID, CourseID, CourseName, Instructor): This table holds information about student enrollments in courses. .
• InstructorDepartment (Instructor, Department): This table associates instructors with their primary department.

Course Enrollment:

InstructorDepartment: