Relations
Last updated
Last updated
A relational database contains multiple tables/relations.
A relation is a set of tuples where each attribute is part of a domain.
Relations are represented as tables.
The columns of the table represent attributes of the relation and the rows/tuples represent the relationship between the entities i.e. the values for the relation.
The following is an example of a relation:
The set of allowed values for an attribute is called the domain of the attribute
Attribute values are atomic i.e. indivisible (they can't be divided into sub-domains)
The special value null is a member of every domain; it means that the value is not available or is unknown
This null value may cause complications in the definition of many operations
Aggregate functions ignore null values
A relational schema consists of a set of attributes and their domains.
where are attributes of the relation.
An relation instance refers to the tuples of a relation at a particular point in time. It is denoted as a table.
Note that relations are unordered i.e. the order in which the tuples are present in the table is irrelevant.
A database consists of multiple relations.
For example, a university database can have relations including student, instructor, advisor, department etc.
It is good to have separate relations instead of combining all the attributes into a single table. If we use a single table to represent everything, there would be two main issues:
redundancy: ex. more than one student having the same instructor
null values: ex. a student who doesn't have an advisor
The following figure illustrates the university database:
Best practices to design "good" relational schemas will be discussed under normalization theory.
