## What is functional dependency in SQL?

A functional dependency (FD) is a relationship between two attributes, typically between the PK and other non-key attributes within a table. For any relation R, attribute Y is functionally dependent on attribute X (usually the PK), if for every valid instance of X, that value of X uniquely determines the value of Y.

**Which of the following rule says if A → B and A → C then A → BC?**

Additive- If A → B and A → C, then A → BC always holds.

**How are functional dependencies determined?**

- A “functional dependency” A->B simply means that no two different values of B are ever related to the same A.
- Since a key must be unique, even if two tuples contain the same value of some attribute(s), the key values must be different nonetheless.

### What are the Armstrong’s inference rules?

The Armstrong’s axioms are:

- Reflexivity rule: if Y ⊆ X then X → Y.
- Augmentation rule: if X → Y then XZ → YZ.
- Transitivity rule: if X → Y and Y → Z then X → Z.

**How do you write a functional dependency from a table?**

A functional dependency is denoted by an arrow “→”. The functional dependency of X on Y is represented by X → Y. Let’s understand Functional Dependency in DBMS with example. In this example, if we know the value of Employee number, we can obtain Employee Name, city, salary, etc.

**What is augmentation rule?**

Augmentation rule − If a → b holds and y is attribute set, then ay → by also holds. That is adding attributes in dependencies, does not change the basic dependencies. Transitivity rule − Same as transitive rule in algebra, if a → b holds and b → c holds, then a → c also holds.

## What do you understand by transitivity rule of functional dependency?

A functional dependency is said to be transitive if it is indirectly formed by two functional dependencies. For e.g. X -> Z is a transitive dependency if the following three functional dependencies hold true: X->Y.

**How are primary keys related to functional dependencies?**

A tables primary key is used to establish relationships with other tables. This is achieved by including the primary key of one table as the Foreign Key of another. The general rule of thumb is to take the primary key of the one side of the relationship and include it in the table of the many side of the relationship.

**What do you mean by inference rules for functional dependencies?**

An inference rule is a type of assertion that a user can apply to a set of functional dependencies to derive other FD (functional dependencies). These axioms in database management system were developed by the William w. Armstrong in 1974.