Lecture 1 - Relational Model

Question 1

Any entity and any relationship are modelled as a …

Answer 1

relation, which maps to a 2D table.

Question 2

What does a 2d table in a relational model have and what do these represent?

Answer 2

• an ordered set of attributes (columns)
• a set of tuples representing instances (rows)
• a specific unique attribute that identifies a tuple in a relation (could be sequential or logical)

Question 3

What is a query?

Answer 3

attributes of interest to be retrieved and constrained attributes to filter out irrelevant or unncessary tuples in the context

Question 4

Bank account context example of a select all query with active accounts that have over 500 pounds?

Answer 4

SELECT name FROM BankAccount
WHERE balance > 500 AND status = ‘active’

Question 5

Formally how is a relation represented?

Answer 5

Rname (attr1 , attr2 , …. , attrn) with each attribute being within a specific domain e.g. int or text

Question 6

What is a schema?

Answer 6

A relation schema can be thought of as the basic information describing a table or relation.

Question 7

What is a relation?

Answer 7

The conceptual model for storing contextual data

Question 8

What is a tuple?

Answer 8

ordered set of values corresponding to attributes of the relation and satisfying the domain constraints

Question 9

Relation vs schema

Answer 9

Relation is the model for an entity and its relationships, it is conceptual. A schema is how the relation will be placed in a table (i.e. table design )

Question 10

What is an instance?

Answer 10

A set of tuples -> populated table / rows in the table

Question 11

What is NULL?

Answer 11

represents an unknown, or inapplicable, or uncertain, or missing value

-> the meaning behind NULL for a certain attribute comes with context

Question 12

Relational Database Schema

Answer 12

set of relations (design of the tables for the whole database (how should each relation be implemented))