Third Normal Form (3NF)

In the journey toward a well-designed relational database, we've conquered First Normal Form (eliminating repeating groups and ensuring atomicity) and Second Normal Form (removing partial dependencies). Yet even after achieving 2NF, a subtle but pervasive form of redundancy can persist—one that arises not from direct relationships with the primary key, but from indirect chains of dependence.

This hidden redundancy manifests through transitive dependencies, where an attribute depends on the primary key through another non-key attribute. Understanding transitive dependencies is the crucial prerequisite for mastering Third Normal Form (3NF), which specifically targets and eliminates these indirect dependency chains.

Before diving into transitive dependencies in database design, let's establish the mathematical concept of transitivity itself. The term derives from the Latin transire, meaning "to go across"—and that's precisely what happens in transitive relationships: properties or implications "travel across" intermediate elements.

Formal Definition of Transitivity:

A relation R on a set S is transitive if and only if:

For all elements a, b, c in S: if aRb and bRc, then aRc

In simpler terms: if a relates to b, and b relates to c, then a must relate to c.

Why Transitivity Matters for Dependencies:

Functional dependencies in databases exhibit transitivity. This is formalized in Armstrong's Axioms through the transitivity rule:

If X → Y and Y → Z, then X → Z

This means that if attribute X functionally determines Y, and Y functionally determines Z, then X also functionally determines Z—through the chain.

The critical insight is this: while X → Z is logically valid, storing Z with X directly (when it's actually determined by Y) creates redundancy. The value of Z is duplicated for every X value that shares the same Y value.

Now we can precisely define what constitutes a transitive dependency in the context of relational database design.

Formal Definition:

Given a relation schema R with attributes and a set of functional dependencies F, a functional dependency X → Z is a transitive dependency if and only if:

1. X → Y holds (where Y is a non-key attribute)
2. Y → Z holds
3. Y ↛ X (Y does not functionally determine X)
4. Z is a non-prime attribute (not part of any candidate key)
5. X → Z is derived through Y (X → Y → Z)

The dependency X → Z is considered "transitive" because it holds transitively through the intermediate attribute Y, rather than directly through a semantic relationship with X.

Breaking Down the Definition:

Let's examine each condition more carefully:

Condition 1 (X → Y): There must be an intermediate attribute Y that X determines. In most cases, X is the primary key or a superkey.

Condition 2 (Y → Z): The intermediate attribute Y must determine the final attribute Z. This is the "transitive step"—Z depends on Y, not directly on X.

Condition 3 (Y ↛ X): If Y could determine X, then X and Y would be equivalent, and we'd have X ↔ Y → Z, which isn't truly transitive—Z would depend on the X/Y equivalence class directly.

Condition 4 (Z is non-prime): If Z were part of a candidate key, removing it through decomposition might lose key information. 3NF specifically targets non-prime attributes.

Condition 5 (Derived through Y): The dependency X → Z exists because of the chain, not because of a direct semantic relationship between X and Z.

To truly understand transitive dependencies, let's dissect a concrete example in complete detail. Consider a relation for managing employees:

Employee Relation:

Functional Dependencies in this Relation:

1. EmpID → EmpName (Direct dependency: Each employee has one name)
2. EmpID → DeptID (Direct dependency: Each employee belongs to one department)
3. DeptID → DeptName (Department determines its name)
4. DeptID → DeptLocation (Department determines its location)
5. EmpID → DeptName (Derived transitively: EmpID → DeptID → DeptName)
6. EmpID → DeptLocation (Derived transitively: EmpID → DeptID → DeptLocation)

Identifying the Transitive Chain:

For the dependency EmpID → DeptName:

• X = EmpID (the primary key)
• Y = DeptID (intermediate attribute, non-key)
• Z = DeptName (final attribute, transitively dependent)
• Check: EmpID → DeptID ✓, DeptID → DeptName ✓, DeptID ↛ EmpID ✓

The Semantic Insight:

The transitive dependency reveals a deeper truth about our data model:

• DeptName is not really an attribute of an Employee—it's an attribute of a Department
• DeptLocation is not really an attribute of an Employee—it's an attribute of a Department
• These attributes appear in the Employee relation only because DeptID is there
• If we change a department's name or location, we must update multiple employee records

This mismatch between logical truth (department attributes belong to departments) and physical storage (department attributes stored with employees) is the essence of the anomalies that transitive dependencies cause.

Transitive dependencies in a relation schema lead to all three classic types of data anomalies. Let's examine each using our Employee example:

Visual representation greatly aids in identifying and communicating transitive dependencies. Several diagramming approaches are commonly used:

Functional Dependency Diagrams:

These diagrams show attributes as nodes and functional dependencies as directed edges. Transitive chains become visually apparent as paths through intermediate nodes.

Dependency Tree View:

Another approach organizes dependencies as a tree where:

• The root is the primary key
• First-level children are directly dependent attributes
• Deeper levels indicate transitive chains

Color Coding Convention:

• 🟢 Green: Primary key / Source of dependencies
• 🟡 Yellow: Intermediate attributes (create transitivity)
• 🔴 Red/Orange: Transitively dependent (problematic)
• ⚪ White/Gray: Direct non-key dependencies (acceptable in 3NF if on full key)

While visual inspection works for small schemas, larger databases require a systematic algorithmic approach to identify transitive dependencies:

Algorithm: Find Transitive Dependencies

Input: Relation R, Set of FDs F, Primary Key K
Output: Set of transitive dependencies TD

1. Compute closure F+ of all functional dependencies
2. Initialize TD = ∅

3. For each FD (X → Z) in F+:
   a. If X is a superkey of R:
      i.  For each attribute Y where Y ⊂ (X)+ and Y ⊄ K:
          - If (Y → Z) ∈ F+ and (Y → X) ∉ F+:
            Add (X → Y → Z) to TD

4. Return TD

Simplified Practical Approach:

For most database design scenarios, use this simpler decision process:

Transitive dependencies appear frequently in real-world database designs. Recognizing common patterns helps you identify them quickly:

Pattern 1: Entity-within-Entity

When one entity is embedded within another's table:

• Order containing Customer details (OrderID → CustomerID → CustomerName)
• Transaction containing Account details (TxnID → AccountNo → BranchCode)
• Prescription containing Doctor details (RxID → DoctorID → Specialty)

Pattern 2: Derived Categorization

When a categorization brings its own attributes:

• Product with Category (ProductID → CategoryID → CategoryDescription)
• Employee with Job Grade (EmpID → GradeCode → GradeSalaryRange)
• Asset with Depreciation class (AssetID → DepreciationClass → DepreciationRate)

Pattern 3: Geographic Hierarchies

Location hierarchies are classic sources:

• Address → ZipCode → City → State → Country
• StoreID → CityID → RegionID → CountryID (each with associated names)

We've established a comprehensive understanding of transitive dependencies—the foundational concept that Third Normal Form directly addresses. Let's consolidate the key insights:

What's Next:

Now that you understand what transitive dependencies are and why they're problematic, we're ready to formally define Third Normal Form (3NF). The next page presents the precise definition of 3NF and explains how it eliminates transitive dependencies while maintaining practical database design constraints.