Mastering Database Management

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Data Modeling

Dependencies in Database Normalization

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In the context of database normalization, dependencies refer to the relationships and interactions between attributes (columns) in a relational database table. Understanding dependencies is crucial for designing normalized database schemas that minimize redundancy and anomalies.

Determinant and Dependent

  • The expression X → Y means ‘if I know the value of X, then I can obtain the value of Y’ (in a table or somewhere).
  • In the expression X → Y, X is the determinant and Y is the dependent attribute.
  • The value X determines the value of Y. The value Y depends on the value of X.

Types of Dependencies:

  • A functional dependency exists when one attribute uniquely determines another attribute in a table.
  • Notation: A → B (Attribute A determines attribute B).

Example: Consider a table representing employees with attributes “EmployeeID” and “EmployeeName.”

Table: Employees

EmployeeIDEmployeeName
1Alice
2Bob
3Charlie

Functional Dependency:

  • EmployeeID → EmployeeName

Explanation:

  • In this example, the attribute “EmployeeName” is functionally dependent on the attribute “EmployeeID.”
  • Knowing the “EmployeeID” uniquely determines the corresponding “EmployeeName.”

A partial dependency occurs when a non-prime attribute depends on only part of the primary key, leading to potential redundancy.

Example: Consider a table representing employees and their assigned projects.

Table: EmployeeProjects

EmployeeIDProjectIDProjectName
1101ProjectA
2102ProjectB
3101ProjectA

Partial Dependency:

  • EmployeeID → ProjectName

Explanation:

  • In this example, “ProjectName” is partially dependent on “EmployeeID.”
  • “ProjectName” depends on part of the primary key (“EmployeeID”) instead of the entire primary key (“EmployeeID,” “ProjectID”).
  • A transitive dependency occurs when an attribute depends on another non-key attribute, which itself depends on the primary key.
  • If A → B and B → C, then A → C is a transitive dependency.

Example: Consider a table representing employees, projects, and project managers.

Table: EmployeeProjects

EmployeeIDProjectIDProjectManager
1101Alice
2102Bob
3101Alice

Transitive Dependency:

  • EmployeeID → ProjectManager
  • ProjectID → ProjectManager

Explanation:

  • In this example, “ProjectManager” is transitively dependent on both “EmployeeID” and “ProjectID.”
  • “ProjectManager” depends on non-key attributes (“EmployeeID” and “ProjectID”).
  • A multi-valued dependency exists when one attribute uniquely determines another attribute, independent of the other attributes.
  • Notation: A →→ B (Attribute A uniquely determines attribute B, independent of other attributes).

Example: Consider a table representing employees and their skills.

Table: EmployeeSkills

EmployeeIDSkillSkillLevel
1JavaIntermediate
1SQLAdvanced
2JavaBeginner
2SQLIntermediate

Multi-valued Dependency:

  • EmployeeID →→ Skill
  • EmployeeID →→ SkillLevel

Explanation:

  • In this example, “Skill” and “SkillLevel” are multi-valued dependent on “EmployeeID.”
  • Knowing “EmployeeID” uniquely determines both the skills and skill levels of an employee.

  • A non-trivial dependency implies that the relationship between attributes is significant and not implied by a candidate key.
  • In a table with a candidate key (A, B), a dependency like A → B is non-trivial.

Example: Consider a table representing courses and their instructors.

Table: Courses

CourseIDInstructor
101Alice
102Bob
103Charlie

Non-trivial Dependency:

  • CourseID → Instructor

Explanation:

  • In this example, the dependency “CourseID → Instructor” is non-trivial.
  • It’s not implied by any candidate key; it’s a significant relationship.

Understanding these dependencies helps in the process of normalization, ensuring that a database is well-organized, minimizes redundancy(duplicacy), and avoids anomalies. Each type of dependency plays a role in determining the structure and integrity of a relational database.

Importance of Dependencies in Database Design:

  1. Database Normalization:
    • Dependencies are the foundation of normalization, a process to minimize data redundancy and anomalies in a database.
    • Normal forms (1NF, 2NF, 3NF, BCNF, 4NF, 5NF) are designed to address specific types of dependencies.
  2. Data Integrity:
    • Understanding dependencies helps ensure data integrity by preventing update, insert, and delete anomalies.
    • Proper normalization reduces the risk of inconsistent or conflicting data.
  3. Query Optimization:
    • Well-designed tables, considering dependencies, lead to efficient query performance.
    • Normalized tables simplify queries by avoiding unnecessary joins and redundancies.
  4. Simplified Schema Design:
    • Knowledge of dependencies helps in creating a more simplified and logical schema design.
    • It provides a clear structure for organizing data based on its inherent relationships.