The Relational Model
Pratt & Adamski, Chapter 2
Relational Algebra
Project
Select
Join
Union
Intersect
Subtract
Product
Division
Relational Database
Based in formal set theory
Most common data structure
Provable design principles
Relational Definition
Relations
Attributes
Relationships
Operations
Relation
Attributes
Occurrences or
Tuples
Attr 1 Attr 2 Attr 3 …
Record 1
Record 2
Record 3
…
Relation Requirements
Attributes are single valued, and all
entries have the same data type
Each attribute has a distinct name
All values in a column are values of the
same attribute
The order of columns is irrelevant
Each row is distinct
The order of rows is irrelevant
Premiere Products
Example
Row Operations:
Select, Project, Join
SELECT returns desired rows
PROJECT returns desired columns
JOIN creates one table from two by
matching specified column values in the
two tables
Set Operations:
Union, Intersection, Difference
Returns the union, intersection, or
difference of rows in two tables
Note: both tables must be “union
compatible.” They must have the same
number of columns and the corresponding
columns must have the same data types.
Table Operations:
Product, Division
PRODUCT (Cartesian product) produces a table
with the columns of both tables and rows
consisting of all combinations of rows from the
two tables. If table A has m rows and table B
has n rows, the product will have m*n rows.
DIVISION (A divided by B where the columns in
B are also in A) produces a table consisting of
columns in A that are not in B and match all the
rows in B.