Relational Models

Domain - The set of allowed values for each attribute

Usually, attribute values are required to be atomic (indivisible). A special value null is part of every domain.

Example: parts of roll numbers in IITB can be used to interpret the branch of that particular student, meaning it isn’t atomic (and is bad practice)

Is null < 5 True or False?

missed

Database Schema - logical structure of database; intructor(ID, name, dept_name, salary)

Database Instance - snapshot of data in database at a given instant

 

Keys

Let $R$ be the set of all attributes in a database. Let $K$ be a subset of $R$.

K is a Superkey of R if values of K are sufficient to identify a unique tuple of each possible relation $r(R)$.

A minimal superkey is called a Candidate key. One of the candidate keys is chosen to be the Primary Key.

Foreign Key Constraint is an integrity constraint which enforces that value in one relation in another. This avoids errors caused by improper data entry.

Example: dept_name in the instructor table can be linked to the dept_name in the department table. Here, instructor is called the Referencing Relation and department is the Referenced Relation.

Referenced Relation should be a primary key.