INFO 340 L05

Read ch 3 and 4
Next week 9,11,12

project deliverable 1 due April 19
see FAQ on website

Assignment 2 updated script for loading tables is available online
due April 21 - Wed

Cardinality - Number of Rows
Degree - Number of Attributes

Every attribute belongs to a domain
A domain specifies the allowable values that an attribute can take
Two or more attributes may share the same domain

Database is nothing but a collection of relations with distinct names

We can thing about tables in terms of mathematical relations and apply set theory

Cartesian product: all the values of relations that can exist

Write the SQL query for:
S = {(x,y)|x ’àà D1, y ’àà D2, y = 3}
S = {(2,3),(4,3)}

Select x, y         //projection
From (D1 X D2)      //cross product take these two relations and make a 3rd relation
Where Y = 3         //selection

Relational Schema
A named relation defined by a set of attribute domain name pairs

Keys are used to uniquely identify rows in a relation

Relational Keys
Superkey
    - Theoretical construct
    - An attribute or a _Set_ of attributes that uniquely identify the row
Candidate Key
    - Minimal number of attributes that uniquely defines a row
    - Two properties:
        * Uniqueness
        * Irreducibility
Primary Key
    - The candidate key that is actually used to identify the tuples
    - Alternative keys are the candidate keys that were not selected
Foreign Key
    - An attribute or a set of attributes that exist in another table
    - Matches a candidate key of some other relation
    
by convention underlined attributes are the primary keys, more than one combine