Functional dependency definition in DBMS Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a functional dependency in a database?

A functional dependency is a relationship between two sets of attributes in a database where the value of one attribute (or set of attributes) uniquely determines the value of another attribute (or set of attributes).

beginner

Explain the notation A → B in functional dependency.

The notation A → B means that attribute A functionally determines attribute B. If two rows have the same value for A, they must have the same value for B.

intermediate

Why are functional dependencies important in database design?

Functional dependencies help identify how data is related and are used to organize tables to reduce redundancy and avoid update anomalies.

beginner

Give a real-life example of a functional dependency.

In a student database, StudentID → StudentName means the student ID uniquely determines the student's name. If two records have the same StudentID, they must have the same StudentName.

intermediate

Can a functional dependency have multiple attributes on the left side? Explain.

Yes, a functional dependency can have a set of attributes on the left side. For example, (CourseID, StudentID) → Grade means the combination of CourseID and StudentID uniquely determines the Grade.

What does the functional dependency A → B mean?

A A and B are unrelated

B B uniquely determines A

C A uniquely determines B

D A and B have the same value

A → B means if two rows have the same value for A, they must have the same value for B.

Which of the following is an example of a functional dependency?

A StudentID → StudentName

B StudentName → StudentID

C Grade → CourseID

D CourseName → StudentID

StudentID uniquely identifies StudentName, so StudentID → StudentName is a functional dependency.

Why are functional dependencies used in databases?

A To increase data duplication

B To organize data and reduce redundancy

C To make queries slower

D To delete data randomly

Functional dependencies help organize data to reduce redundancy and avoid anomalies.

Can a functional dependency have multiple attributes on the left side?

A Only in special databases

B No, only one attribute is allowed

C Only if the right side has multiple attributes

D Yes, it can have a set of attributes

Functional dependencies can have multiple attributes on the left side, like (A, B) → C.

If two rows have the same value for attribute A, what must be true for attribute B in A → B?

A They must have the same value for B

B They can have different values for B

C B must be null

D B must be unique

In A → B, same A means same B values to maintain the dependency.

Describe what a functional dependency is and why it matters in database design.

Think about how one piece of data can decide another.

Give an example of a functional dependency from everyday life and explain it.

Consider IDs and names or similar pairs.

Practice

1. What does the functional dependency X -> Y mean in a database?

easy

A. Knowing X allows you to find Y uniquely

B. X and Y are unrelated data sets

C. Y determines X uniquely

D. X and Y are always equal

Solution

1. Step 1: Understand the notation X -> Y

   This notation means that the value of X determines the value of Y uniquely in the database.

2. Step 2: Interpret the meaning in simple terms

   If you know X, you can find Y without ambiguity. This is the core idea of functional dependency.

3. Final Answer:

   Knowing X allows you to find Y uniquely -> Option A

4. Quick Check:

   Functional dependency means X determines Y [OK]

Hint: Remember: X determines Y means knowing X gives Y [OK]

Common Mistakes:

• Confusing which attribute determines which
• Thinking X and Y are unrelated
• Assuming dependency means equality

2. Which of the following is the correct way to write a functional dependency?

easy

A. X -> Y

B. X = Y

C. X <-> Y

D. X + Y

Solution

1. Step 1: Recall the standard notation for functional dependency

   The correct notation uses an arrow pointing from X to Y, written as X -> Y.

2. Step 2: Eliminate incorrect options

   X = Y means equality, X <-> Y suggests bidirectional dependency, and X + Y is not a valid notation here.

3. Final Answer:

   X -> Y -> Option A

4. Quick Check:

   Functional dependency uses arrow notation [OK]

Hint: Look for arrow notation X -> Y for functional dependency [OK]

Common Mistakes:

• Using equals sign instead of arrow
• Confusing bidirectional with functional dependency
• Using plus sign mistakenly

3. Given a table with columns StudentID and StudentName, which functional dependency is correct?

medium

A. StudentName -> StudentID

B. StudentID -> StudentName

C. StudentID -> StudentID

D. StudentName -> StudentName

Solution

1. Step 1: Identify the unique identifier

   Usually, StudentID uniquely identifies a student, so it determines StudentName.

2. Step 2: Check the direction of dependency

   Knowing StudentID gives StudentName, but not necessarily the other way around.

3. Final Answer:

   StudentID -> StudentName -> Option B

4. Quick Check:

   Unique ID determines name [OK]

Hint: Unique keys determine other attributes [OK]

Common Mistakes:

• Assuming name uniquely identifies ID
• Confusing attribute roles
• Ignoring uniqueness of StudentID

4. Identify the error in this functional dependency statement: EmployeeName -> EmployeeID where EmployeeID is unique.

medium

A. EmployeeName must be numeric

B. EmployeeID cannot be unique

C. Functional dependency must be bidirectional

D. EmployeeName may not uniquely determine EmployeeID

Solution

1. Step 1: Understand uniqueness of EmployeeID

   EmployeeID is unique, so it can determine EmployeeName, but not vice versa.

2. Step 2: Analyze the given dependency

   EmployeeName may repeat for different employees, so it cannot uniquely determine EmployeeID.

3. Final Answer:

   EmployeeName may not uniquely determine EmployeeID -> Option D

4. Quick Check:

   Non-unique attribute cannot determine unique ID [OK]

Hint: Only unique attributes determine others [OK]

Common Mistakes:

• Assuming all attributes are unique
• Thinking dependency is always two-way
• Confusing attribute types

5. In a database, if A -> B and B -> C, what can we conclude about the functional dependency between A and C?

hard

A. B -> A

B. C -> A

C. A -> C

D. C -> B

Solution

1. Step 1: Understand transitive dependency

   If A determines B, and B determines C, then A determines C by transitivity.

2. Step 2: Apply the rule to given dependencies

   Since A -> B and B -> C, it follows that A -> C.

3. Final Answer:

   A -> C -> Option C

4. Quick Check:

   Functional dependencies are transitive [OK]

Hint: Chain dependencies to find indirect ones [OK]

Common Mistakes:

• Reversing dependency direction
• Ignoring transitive property
• Confusing attributes order