Database Concepts

Lesson

Part 1 — why we need databases, what a DBMS does, and the relational model: tuples, attributes, domains, degree and cardinality.

In this lesson you will

Explain the problems of keeping data in separate files and how a DBMS solves them
Use the terms relation, tuple, attribute and domain correctly
Find the degree and cardinality of a relation, after changes, and of a Cartesian product

Part 1 at a glance

Concept

Data kept in separate files

Before databases, each office kept its own file. The same student details were typed again and again.

The same facts typed into three files.
File (imagined school)	What it stores	Repeated data
Admission file	AdmNo, name, class, address, phone	—
Fee file	AdmNo, name, class, month, amount	name, class
Transport file	AdmNo, name, address, phone, bus route	name, address, phone

Concept

Table of five file-system problems: redundancy, inconsistency, unavailability, unauthorised access and data dependence, with what goes wrong and an example from an imagined school
Problem	What goes wrong	In the imagined school
Redundancy	Same data stored many times	Name typed in three files
Inconsistency	Copies disagree after an update	New phone number in one file only
Unavailability	Data exists but is hard to get	Bus users with unpaid fees takes days
Unauthorised access	Anyone with the file sees everything	Fee clerk can read health notes
Data dependence	Programs break when the file layout changes	A new column breaks the fee program