The Relational Model

Quick Informal Defs for Relational Model

Relation: a table of values, with:

• rows where elements represent real-world facts
• columns which represent a property

A key of a relation:

• makes a row uniquely identifiable
• may be an assigned sequential number (artificial/surrogate key)

A schema is a description of a relation:

• name of relation
• name of attributes of relation
• Example: Students(StudentId, StudentName, Grade, Age) describes relation Students

Key Facts

Relational Model

A relation is a table of values, like below:

oStudentID	Grade	Age

A relation is a set of tuples including:

• rows which are called tuples (denoted by $<\cdots >$ in CS258)
• columns, called attributes

Each attribute:

• has a set of valid values (or a domain)
  • For example, the domain of Grade may be $\{0,1,2,\dots ,100\}$
  • Domains also often have formats

Each tuple:

• has data elements that correspond to real-world data
  • has a value from the domain of the attribute
• is uniquely identifiable (by the key)
  • An assigned ID (which can be a sequential number) are called artificial or surrogate

The set of rows is also called a relation’s state.

• state $\subseteq a_1\times a_2\times \cdots \times a_n$ where $a_i$ is the domain of the corresponding attribute.

You might ask about the headers, since they appear to be the first tuple in the relation.
The headers are called attribute names, and they are for interpreting the data elements.

Schema

The Schema of a relation is a description, in the form $RELATION(A1,A2,\dots ,A_n)$.
For example, the schema for the above relation is $STUDENT(StudentID,Grade,Age)$.

Relational DB Constraints

Key Constraints

Entity Integrity Constraints

Why should all info not be stored in one table?
- Lots of redundant data