relational model

introduced by E. F. Codd in 1970, is based on predicate logic and set theory.

Predicate logic

s used extensively in mathematics to provide a framework in which an assertion (statement of fact) can be verified as either true or false.

Set theory

is a part of mathematical science that deals with sets, or groups of things, and is used as the basis for data manipulation in the relational model.

table (relation)

is as a two-dimensional structure composed of rows and columns.

table row (tuple)

represents data about an entity

attribute

Each table column represents an

domain

Each column has a specific range of values known as the attribute

key

is an attribute or group of attributes that determines the values of other attributes.

Determination

is the state in which knowing the value of an attribute makes it possible to determine the value of another. It is based on the relationships among the attributes.

Functional dependence

means that the value of one or more attributes determines the value of one or more other attributes

determinant or the key

The attribute whose value determines another is called the

ATT_A → ATT_B

The standard notation for representing the relationship between attributes is:

composite key

is a key that is composed of more than one attribute.

Superkey

An attribute or combination of attributes that uniquely identifies any row in the table

candidate Key

A superkey without any unnecessary attributes

Primary Key

A candidate key selected to uniquely identify all other attribute values in any given row; cannot contain null entries.

Foreign Key

An attribute or combination of attributes in one table whose values must either match the primary key in another table or be null.

Secondary Key

An attribute or combination of attributes used strictly for data retrieval purposes.

flags

To avoid nulls, special codes called __ are used to indicate the absence of some value

Relational algebra

a is a set of mathematical principles that form the basis for manipulating relational table contents

closure

The use of relational algebra operators on existing relations (tables) produces new relations is called

predicate

The condition to be evaluated is also known as

SELECT σ

Retrieves a subset of rows

PROJECT π

Retrieves a subset of columns

UNION U

Merges two union-compatible tables into a new table, dropping the duplicate rows

INTERSECT ∩

Retrieves rows that are common to two union-compatible tables

union-compatible

Two or more tables that have the same number of columns and the corresponding columns have compatible domains are

DIFFERENCE

Retrieves rows from one table that are not found in another union-compatible table

PRODUCT

Retrieves possible pairs of rows from two tables (Cartesian Product)

JOIN ⨝

Retrieves rows from two tables based on criteria

DIVIDE

Retrieves values