Relational Algebra 关系代数

Relational Algebra

Relational Algebra is the mathematical basis for the query language SQL

Introduction.

So now you have learn how to design good conceptual models to store information with the ER-model
And you also know how to turn an ER-model into a Relational model

Let us jump the next step – which is defining the Relational Database and entering the data into the database – and look at data manipulation

What can we do with the data ????

Relational Algebra: Overview

is a mathematical language for manipulating relations (ala mathematical definition of “relation” – which is a subset of some cartesian product)

contains:

set operators
relational database specific operators
set functions

is used to specify a result set that satisfies a certain “selection criteria”

the result of an operation in relational algebra is always a relation

Set operations

You should all know what set union is:

Suppose A = {i, j, k} and B = {k, x, y}
Then the union of A and B is {i, j, k, x, y}

Note that tuples in sets A and B must have the same FORMAT (same number and type of attributes) to be used in the union operation.

You should also have learned what set intersection is:

Suppose A = {i, j, k} and B = {k, x, y}
Then the intersection of A and B is {k}

Note that tuples in sets A and B must have the same FORMAT (same number and type of attributes) to be used in the intersection operation.

You should also have learned what set difference is:

Suppose A = {i, j, k} and B = {k, x, y}
Then the difference of A and B is {i, j}

Note that tuples in sets A and B must have the same FORMAT (same number and type of attributes) to be used in the set difference operation.

I have already discussed the cartesian product: click here 

Set Functions

Operates on a set of values and produce a single value

To illustrate the various set function, we apply the set functions on this set of value:

A = {1, 4, 9}

Results:

Relational database specific operations

σ _(Condition) (R)         (selection)

Selects tuples from a relation R that satisfies the condition Condition
The Condition expression contains constants and/or attributes of relation R (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

Examples:

Retrieve all employee tuples working for department number 4:

The following diagram shows the result graphically (hopefully, it will illustrates the concept unambiguously):

Retrieve all employees earning more than $30,000:

Retrieve all employees working for department number 4 and eaning more than $30,000:

OR:

π _{(attribute-list)}         (projection)

Selects out only the attribute values 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

Examples:

Retrieve all SSN from the employees

The following diagram shows the result graphically (hopefully, it will illustrates the concept unambiguously):

Retrieve the sex information from all employees:

The following diagram shows the result graphically (hopefully, it will illustrates the concept of projection unambiguously):

Notes:

Combining information from 2 relations

Consider the following query:

Observation:

But we must be careful in combining the information:

   Employee:
   +-----------+--------+---------+-----------+-----+ 
   | ssn       | fname  | lname   | bdate     | dno | .... (other columns omitted)     
   +-----------+--------+---------+-----------+-----+ 
   | 123456789 | John   | Smith   | 09-JAN-55 |   5 | 
   | 333445555 | Frankl | Wong    | 08-DEC-45 |   5 | 
   | 999887777 | Alicia | Zelaya  | 19-JUL-58 |   4 | 
   | 987654321 | Jennif | Wallace | 20-JUN-31 |   4 | 
   | 666884444 | Ramesh | Narayan | 15-SEP-52 |   5 | 
   | 453453453 | Joyce  | English | 31-JUL-62 |   5 | 
   | 987987987 | Ahmad  | Jabbar  | 29-MAR-59 |   4 | 
   | 888665555 | James  | Borg    | 10-NOV-27 |   1 | 
   +-----------+--------+---------+-----------+-----+ 

   Department:
   +----------------+---------+-----------+--------------+
   | dname          | dnumber | mgrssn    | mgrstartdate |
   +----------------+---------+-----------+--------------+
   | Research       |       5 | 333445555 | 22-MAY-78    |
   | Administration |       4 | 987654321 | 01-JAN-85    |
   | Headquarters   |       1 | 888665555 | 19-JUN-71    |
   +----------------+---------+-----------+--------------+

How to “combine” tuples from Employee and Department:

Solution:

First find the department number of the “Research” department:

Graphically:

Then we combine the department and employee through a “mindless” cartesian product operation:

The result of this operation is “tagging” an extra column “dnumber” to the employee relation:

Now we can use a select operation to select out the “right tuples” :

Because dnumber is now an attribute of the (new) relation R₂ !!!

Graphically:

Efficiency consideration of queries

Correctness is the most important property of a query

The second most important property of a query is efficiency

There maybe multiple queries that retrieve the correct set of tuples (answer)

The difference of the queries may be their execution speed

Consider the following solution:

Although this query is correct , it will take longer time to process because of the larger amount of intermediate data that the query produces:

The cartesian product employee x department will produce huge number of tuples – many more than in the previous solution.

Join operations

The join operation (bow tie symbol) is equivalent to:

Example:

Retrieve all employees from the “Research” department

slower (but correct) solution:

It is slower because the join operation will result in more tuples as intermediate results

The faster version is:

The “theta join” operation

The condition in the join operation is of the form:

   Condition

 and Condition

 and ....       
   

Each condition is of the form “x θ y”, where x and y are attributes or constants

And θ is one of the following relational operators:

A join operation with a “general” join condition (where θ can be any relational operation), is called a theta join

The “equi-join” operation

Often (almost always :-)), we use the equal relational operation in the join operation:

     Condition

 and Condition

 and ....     

where each condition is of the form:

    x == y        

where x and y are attributes or constants

Equi-join:

The “natural join” (*) operation

The natural join operation is an equi-join operation follow by a projection operation that removes one of the duplicate attributes.

Since the equi-join will for tuples when the attribute values are equal, all tuples that produced by an equi-join will have the same attribute value under the join condition attributes.

The natural join operation is denoted by a star (*):

    R1 * attr1,attr2 R2       

Outer-join

Outer-joins:

The 3 flavors of outer-joins:

     A:                 B:

        X     Y            U     V
     +-----+-----+      +-----+-----+                  
     |  1  |  7  |      |  1  |  8  |
     +-----+-----+      +-----+-----+
     |  2  |  5  |      |  2  |  3  |
     +-----+-----+      +-----+-----+
     |  3  |  4  |      |  4  |  9  |
     +-----+-----+      +-----+-----+

  A ⟕A.X=B.U B:

        X     Y     U     V
     +-----+-----+-----+-----+ 
     |  1  |  7  |  1  |  8  |
     +-----+-----+-----+-----+
     |  2  |  5  |  2  |  3  |
     +-----+-----+-----+-----+
     |  3  |  4  | NULL| NULL|
     +-----+-----+-----+-----+

     A:                 B:

        X     Y            U     V
     +-----+-----+      +-----+-----+                  
     |  1  |  7  |      |  1  |  8  |
     +-----+-----+      +-----+-----+
     |  2  |  5  |      |  2  |  3  |
     +-----+-----+      +-----+-----+
     |  3  |  4  |      |  4  |  9  |
     +-----+-----+      +-----+-----+

  A ⟖A.X=B.U B:

        X     Y     U     V
     +-----+-----+-----+-----+ 
     |  1  |  7  |  1  |  8  |
     +-----+-----+-----+-----+
     |  2  |  5  |  2  |  3  |
     +-----+-----+-----+-----+
     | NULL| NULL|  4  |  9  |
     +-----+-----+-----+-----+

     A:                 B:

        X     Y            U     V
     +-----+-----+      +-----+-----+                  
     |  1  |  7  |      |  1  |  8  |
     +-----+-----+      +-----+-----+
     |  2  |  5  |      |  2  |  3  |
     +-----+-----+      +-----+-----+
     |  3  |  4  |      |  4  |  9  |
     +-----+-----+      +-----+-----+

  A ⟗A.X=B.U B:

        X     Y     U     V
     +-----+-----+-----+-----+ 
     |  1  |  7  |  1  |  8  |
     +-----+-----+-----+-----+
     |  2  |  5  |  2  |  3  |
     +-----+-----+-----+-----+
     |  3  |  4  | NULL| NULL|
     +-----+-----+-----+-----+
     | NULL| NULL|  4  |  9  |
     +-----+-----+-----+-----+

Note:

     A:                 B:

        X     Y            X     V
     +-----+-----+      +-----+-----+                  
     |  1  |  7  |      |  1  |  8  |
     +-----+-----+      +-----+-----+
     |  2  |  5  |      |  2  |  3  |
     +-----+-----+      +-----+-----+
     |  3  |  2  |      |  4  |  9  |
     +-----+-----+      +-----+-----+

  A ⟗ B:

        X     Y     V
     +-----+-----+-----+ 
     |  1  |  7  |  8  |
     +-----+-----+-----+
     |  2  |  5  |  3  |
     +-----+-----+-----+
     |  3  |  2  | NULL|
     +-----+-----+-----+
     |  4  | NULL|  9  |
     +-----+-----+-----+

Teaching notes:

More examples….

Find fname and lname of all employees working in the “Research” department that earn more than $50,000

Find fname and lname of John Smith’s supervisor

Find fname and lname of all employees that have dependents

Find fname and lname of all employees that do not have dependents

This solution is wrong:

The following diagram shows how the query works on an example database, which illustrates why the query is wrong:

The correct solution is:

Help1 = set of SSN of employees with dependents
Help2 = set of SSN of employees without any dependents

Find fname and lname of employees who has more than one dependents of the same sex (i.e., 2 or more boys or 2 or more girls):

NOTE: so join conditions do not always have to be “equal” !!!

A comment…

Clearly, I can easily formulate a gad-sillion different queries in the next day or so, so:

DO NOT

 prepare for tests with 

memorizing
 facts – you gotta understand what’s going on and do the queries “from scratch”….

And for those students that have never had me before: I have a reputation to challenge student’s grey cells to work overtime…. so you should expect some very interesting queries…. and plenty of opportunity to practice your skill (in the homeworks and SQL projects).

http://www.mathcs.emory.edu/~cheung/Courses/377/Syllabus/4-RelAlg/intro.html