- Syntax:
σ(Condition)(R) - Effect:
- Selects
tuples from a relation
R that satisfies the
condition
Condition
- The Condition expression contains:
- constants and/or
- attributes of relation R
- However: the condition expression cannot be a set !!!
- The result of σ (Condition) (R) is a subset of tuples of R that satisfies the boolean condition Condition
- Selects
tuples from a relation
R that satisfies the
condition
Condition
- Example 1:
- Retrieve all employee tuples
for employees that work for
department number 4:
Answer:
σ (dno = 4) (employee) - The following diagram shows the result graphically
(hopefully, it will illustrates the concept
unambiguously):
- Retrieve all employee tuples
for employees that work for
department number 4:
- Example 2:
- Retrieve all employees earning more than $30,000:
Answer:
σ (salary > 30000) (employee)
- Retrieve all employees earning more than $30,000:
- Example 3:
- Retrieve all employees working for department number 4 and
eaning more than $30,000:
Answer:
σ (dno = 4 and salary > 30000) (employee) OR:
σ (salary > 30000) ( σ (dno = 4) (employee) )
- Retrieve all employees working for department number 4 and
eaning more than $30,000:
- Syntax:
π (attribute-list) (R)
Effect:
- "Selects out"
only
the attribute values
given in the
attribute-list
from all tuples in relation R
- The attribute-list contains the
list of attributes
in relation R that will be selected.
- The result of π (attribute-list) (R) contains the is a subset of tuples of R that satisfies the boolean condition Condition
- "Selects out"
only
the attribute values
given in the
attribute-list
from all tuples in relation R
- Example 1:
- Retrieve only the
SSN field from the employees
π ssn (employee) - The following diagram shows the result graphically
(hopefully, it will illustrates the concept
unambiguously):
- Example 2:
- Retrieve the sex information from all employees:
π sex (employee) The following diagram shows the result graphically (hopefully, it will illustrates the concept of projection unambiguously):
Notes:
- The output
of a Relational Algebra
query is set.
A set in Mathematics does not have duplicate elements
Therefore, the result of πsex (employee) is the set {F, M} because duplicates are removed
- In SQL,
the user will have the
option
to remove duplicates
That is because removing duplicates require longer time to process....
(The user has the option to incur the cost or not...)
- Retrieve the sex information from all employees:
- Retrieve only the
SSN field from the employees
- Recall the
data manipulation languages:
- Relational Algebra
- Relational Calculus
- SQL
- Characteristics of
data manipulation languages:
- Data manipulation langugaes can express (specify) what (data) a user wants
Examples:
- Retrieve the
names of the
female employees
Answer:
- πfname, lname ( σsex = 'F' ( employee ) )
Example using a specific Employee relation:
- Retrieve the
names of the
employees who
earn more than $50,000
Answer:
- πfname, lname ( σsalary > 50000 ( employee ) )
Example using a specific Employee relation:
- Generations of
Programming Languages:
- 1st generation
programming language:
- The earliest way to
program a computer:
- By flipping switches (= bits (0/1) !!!)
- You write a computer program as a sequence of bit patterns
- The earliest way to
program a computer:
- 2nd generation
programming languages:
- You write a computer program using assembler (nmemonic) codes
- 3rd generation
programming languages:
- You write a computer program
using procedural statements such as:
- Assignment statement
- Conditional statements
- Repetition statemanets
- Procedural languages expresses
how to get things done
Examples:
- Java, C, Python, ....
- You write a computer program
using procedural statements such as:
- 4th generation
programming languages:
- You express what you want
in a 4th generation language
(and leave
the how to get it done to
the compiler)
Examples:
- Relational Algebra
- Relational Calculus
- SQL
Wikipedia: click here
- You express what you want
in a 4th generation language
(and leave
the how to get it done to
the compiler)
- 1st generation
programming language: