CSC 280: INTRODUCTION TO DATABASE MANAGEMENT SYSTEMSDr. Adam Anthony

Fall 2012

Lecture 2

Introduction to the Relational Model Views of Data Definition of a Relation Schemas and Keys

Different Views of Data

Disk (physical) level: Where does each bit go on the drive, and how

do we retrieve it when we need it? Logical level:

What is the picture in our head of how the data is arranged?

May or may not differ from reality of the disk level

View level: That portion of the logical level that we can see Sometimes a refined version of the logical level

What is a Relation?

UNLEARN: In a database context, a relation is NOT a link between two different entities (e.g. a family tree relation) Though we can represent such data

RE-LEARN: A relation is actually a theoretical collection of potential entities with common attributes Imagine the data to describe a single instructor Imagine a relation for instructor as being a data

table, but you don’t know what instructors are listed in that table yet Any instructor who ever exists MIGHT be in the table

A Relation Instance

Given a Relational Definition: Instructor(ID, name, dept_name, salary)

We create a data file with actual data:

IMPORTANT: OPERATIONS DEFINED ON RELATIONS WILL BE INDEPENDENT OF ANY ACTUAL DATA

In-Class Exercise

Same groups as last time: Use the relation schema definition format:

relation-name(attribute1,attribute2,…)

To define at least three relations you might see in a music database

Terminology

Relation: a data table with defined column types Tuple: a single row in a relation table Attribute: a single column in a relation or tuple Relation Instance: a data table with specific data

entries already in it Order-Independent: the rows in a relation are not

ordered; if the tuples are the same but in different order, the relations are equal.

Domain: indicates the kind of values allowed for an attribute (integer range, characters, date/time, etc) Should be atomic, or, single-valued

Keys

Let K R K is a superkey of R if values for K are sufficient to

identify a unique tuple of each possible relation r(R) Example: {ID} and {ID,name} are both superkeys of

instructor.

Superkey K is a candidate key if K is minimalExample: {ID} is a candidate key for Instructor

One of the candidate keys is selected to be the primary key. which one?

Find the Keys

Determine good primary keys for part of the university schema (underline your choice): Instructor(ID,name,dept_name,salary) Department(dept_name,building,budget) Student(ID,name,dept_name,tot_cred) Advisor(s_id,i_id) Takes(ID,course_id,sec_id,semester,year,grade) Classroom(building,room_number,capacity) Time_slot(time_slot_id,day,start_time,end_time)

Foreign Keys

Foreign key constraint: Value in one relation must appear in another Referencing relation Referenced relation

Textbook University Schema Diagram

In-Class Activity

Draw a schema diagram for the music database from earlier. Underline primary keys, and use arrows for foreign keys.