Relational Algebra

What does a relational database represent?

data as a collection of related relations

What is meant by the domain?

permitted values for an attribute

What is meant by degree?

the number of attributes

What is meant by cardinality in terms of tuples?

the number of tuples

How is a function mapped?

each input is related to exactly one output

What is a relation?

a set of ordered pairs (one input can relate to several outputs)

What is the definition for a relation/table?

a logical representation of data based on set theory and predicate logic

What are the 3 rules of a relation?

1. single valued cells

2. unique rows

3. row + column order don’t matter

What are the 3 aspects of data that the relational model covers?

1. Data Structure - table,column,row,domain

2. Data Integrity - rules to ensure data is valid

3. Data Manipulation - use relational algebra to retrieve + change data

What does the relational model consist of?

• data structures

  • entities + their relations

• data constraints

  • restrication on data, relations… etc.

• relational operators

  • restrict, project, cartesian product, union, difference

What are the 4 objectives of the relational model?

1. Data Indepence - allow changes w/o affecting applications that use it

2. Control Redundancy - data not stored in multiple places

3. Maintain Consistency - copies are accurate and up to date

4. Enable Set-Oriented Languages (e.g. relational algebra)

What is a relational schema?

a textual representation for a data model

Relational Vocab: RELATION

the whole table

Relational Vocab: TUPLE

1 row

Relational Vocab: ATTRIBUTE

1 named column

What is an alternate key?

a candidate key that was not chosen as the primary key

What is a foreign key?

attribute(s) that refer to a candidate key in another relation

What do NULL values represent?

missing information (e.g. unknown value, value inserted later)

What are the types of database constraints?

• entity integrity - each row is unique + identifiable

• referential integrity - accuracy + consistency of data between tables

• multiplicity

• required data (primary key not null)

• domain constraints

What is the difference between entity and referential integrity?

ENTITY INTEGRITY

• constraint on PK

• unique values + not null

REFERENTIAL INTEGRITY

• constrain on FK

• values match candidate key referred to/ be null