ics 541 - 01 (072)relational data model 11 database systems: a review relational data model 1 dr....

ICS 541 - 01 (072) Relational Data Model 1 1

Database Systems: A Review

Relational Data Model 1

Dr. Muhammad Shafique


Outline• Relational data model

1. Data Management• Informal definitions

• Formal definitions

• Integrity constraints

• Update operations

2. Data Manipulation• Relational algebra

• Relational calculus• Tuple calculus

• Domain calculus

3. Implementation• SQL


References

• Textbook Chapter 5, 6, 8 The Relational Data Model and Relational Database Constraints

• Textbook Chapter 6 The Relational Algebra and Relational Calculus

• Textbook Chapter 8: SQL 99• E.F. Codd, "A Relational Model for Large Shared

Data Banks," Communications of the ACM, June 1970


1. Data management


Relational Model Concepts

• The relational Model of Data is based on the concept of a Relation• The strength of the relational approach to data management

comes from the formal foundation provided by the theory of relations

• Note: There are several important differences between the formal model and the practical model

• A “relation” is a fundamental mathematical notion expressing a relationship between elements of sets. • A binary relation from a set A to a set B is a subset

R A × B.



• Definitions of relation1. A relation L is defined by specifying two mathematical objects as its

constituent parts:1. The first part is called the figure of L, notated as figure(L) or F(L).

2. The second part is called the ground of L, notated as ground(L) or G(L).

2. A k-ary relation L over the nonempty sets X1, …, Xk is a (1+k)-tuple L = (F(L), X1, …, Xk) where F(L) is a subset of the Cartesian product X1 × … × Xk. If all of the Xj for j = 1 to k are the same set X, then L is more simply called a k-ary relation over X. The set F(L) is called the figure of L and, providing that the sequence of sets X1, …, Xk is fixed throughout a given discussion or determinate in context, one may regard the relation L as being determined by its figure F(L).



• A Relation is a mathematical concept based on the ideas of sets

• The model was first proposed by Dr. E.F. Codd of IBM Research in 1970 in the following paper:• "A Relational Model for Large Shared Data Banks,"

Communications of the ACM, June 1970

• The above paper caused a major revolution in the field of database management and earned Dr. Codd the coveted ACM Turing Award


Informal Definitions

• Informally, a relation looks like a table of values.

• A relation typically contains a set of rows.

• The data elements in each row represent certain facts that correspond to a real-world entity or relationship• In the formal model, rows are called tuples

• Each column has a column header that gives an indication of the meaning of the data items in that column• In the formal model, the column header is called an attribute name (or

just attribute)


Example of a Relation


Informal Definitions

• Key of a Relation:• Each row has a value of a data item (or set of items) that

uniquely identifies that row in the table• Called the key

• Sometimes row-ids or sequential numbers are assigned as keys to identify the rows in a table

• Called artificial key or surrogate key


Formal Definitions - Schema

• The Schema (or description) of a Relation:• Denoted by R(A1, A2, .....An)

• R is the name of the relation

• The attributes of the relation are A1, A2, ..., An

• Example:

CUSTOMER (Cust-id, Cust-name, Address, Phone#)• CUSTOMER is the relation name

• Defined over the four attributes: Cust-id, Cust-name, Address, Phone#

• Each attribute has a domain or a set of valid values. • For example, the domain of Cust-id is 6 digit numbers.


Formal Definitions - Tuple

• A tuple is an ordered set of values (enclosed in angled brackets ‘< … >’)

• Each value is derived from an appropriate domain.

• A row in the CUSTOMER relation is a 4-tuple and would consist of four values, for example:• <632895, "John Smith", "101 Main St. Atlanta, GA 30332", "(404)

894-2000">

• This is called a 4-tuple as it has 4 values

• A tuple (row) in the CUSTOMER relation.

• A relation is a set of such tuples (rows)


Formal Definitions - Domain

• A domain has a logical definition:• Example: “Cell_phone_numbers” are the set of 10 digit phone numbers

• A domain also has a data-type or a format defined for it.• The Cell_phone_numbers may have a format: ddd-ddd-dddd where each d is a

decimal digit.• Dates have various formats such as year, month, date formatted as yyyy-mm-

dd, or as dd mm,yyyy etc.

• The attribute name designates the role played by a domain in a relation:• Used to interpret the meaning of the data elements corresponding to that

attribute• Example: The domain Date may be used to define two attributes named

“Invoice-date” and “Payment-date” with different meanings


Formal Definitions - State

• The relation state is a subset of the Cartesian product of the domains of its attributes• each domain contains the set of all possible values the

attribute can take.

• Example: attribute Cust-name is defined over the domain of character strings of maximum length 25• dom(Cust-name) is varchar(25)

• The role these strings play in the CUSTOMER relation is that of the name of a customer.


Formal Definitions - Summary

• Formally,• Given R(A1, A2, .........., An)

• r(R) dom (A1) X dom (A2) X ....X dom(An)

• R (A1, A2, …, An) is the schema of the relation

• R is the name of the relation

• A1, A2, …, An are the attributes of the relation

• r(R): a specific state (or "value" or “population”) of relation R – this is a set of tuples (rows)• r(R) = {t1, t2, …, tn} where each ti is an n-tuple

• ti = <v1, v2, …, vn> where each vj element-of dom(Aj)


Formal Definitions - Example

• Let R(A1, A2) be a relation schema:

• Let dom(A1) = {0,1}

• Let dom(A2) = {a,b,c}

• Then: dom(A1) X dom(A2) is all possible combinations:

{<0,a> , <0,b> , <0,c>, <1,a>, <1,b>, <1,c> }

• The relation state r(R) dom(A1) X dom(A2)

• For example: r(R) could be {<0,a> , <0,b> , <1,c> }• This is one possible state (or “population” or “extension”) r of the

relation R, defined over A1 and A2.

• It has three 2-tuples: <0,a> , <0,b> , <1,c>


Definition Summary

Informal Terms Formal Terms

Table Relation

Column Header Attribute

All possible Column Values Domain

Row Tuple

Table Definition Schema of a Relation

Populated Table State of the Relation


Example – A relation STUDENT


Characteristics Of Relations

• Ordering of tuples in a relation r(R):• The tuples are not considered to be ordered

• Ordering of attributes in a relation schema R (and of values within each tuple):• We will consider the attributes in R(A1, A2, ..., An) and

the values in t=<v1, v2, ..., vn> to be ordered .• However, a more general alternative definition of relation does not

require this ordering.


Same state as previous Figure (but with different order of tuples)



• Values in a tuple:• All values are considered atomic (indivisible).

• Each value in a tuple must be from the domain of the attribute for that column

• If tuple t = <v1, v2, …, vn> is a tuple (row) in the relation state r of R(A1, A2, …, An)

• Then each vi must be a value from dom(Ai)

• A special null value is used to represent values that are unknown or inapplicable to certain tuples.



• Notation:• We refer to component values of a tuple t by:

• t[Ai] or t.Ai

• This is the value vi of attribute Ai for tuple t

• Similarly, t[Au, Av, ..., Aw] refers to the subtuple of t containing the values of attributes Au, Av, ..., Aw, respectively in t


Relational Integrity Constraints

• Constraints are the conditions that must hold on all valid relation states.

• There are three main types of constraints in the relational model:• Key constraints

• Entity integrity constraints

• Referential integrity constraints

• Another implicit constraint is the domain constraint• Every value in a tuple must be from the domain of its attribute (or it

could be null, if allowed for that attribute)


Key Constraints

• Superkey of R: • It is a set of attributes SK of R with the following condition:

• No two tuples in any valid relation state r(R) will have the same value for SK

• That is, for any distinct tuples t1 and t2 in r(R), t1[SK] t2[SK]

• This condition must hold in any valid state r(R)

• Key of R:• A "minimal" superkey

• That is, a key is a superkey K such that removal of any attribute from K results in a set of attributes that is not a superkey (does not possess the superkey uniqueness property)


Key Constraints (continued)

• Example: Consider the CAR relation schema:• CAR(State, Reg#, SerialNo, Make, Model, Year)

• CAR has two keys:• Key1 = {State, Reg#}

• Key2 = {SerialNo}

• Both are also superkeys of CAR

• {SerialNo, Make} is a superkey but not a key.

• In general:• Any key is a superkey (but not vice versa)

• Any set of attributes that includes a key is a superkey

• A minimal superkey is also a key


Key Constraints (continued)

• If a relation has several candidate keys, one is chosen arbitrarily to be the primary key. • The primary key attributes are underlined.

• Example: Consider the CAR relation schema:• CAR(State, Reg#, SerialNo, Make, Model, Year)• We chose SerialNo as the primary key

• The primary key value is used to uniquely identify each tuple in a relation• Provides the tuple identity

• Also used to reference the tuple from another tuple• General rule: Choose as primary key the smallest of the candidate keys

(in terms of size)• Not always applicable – choice is sometimes subjective


CAR table with two candidate keys – LicenseNumber chosen as Primary Key


Relational Database Schema

• Relational Database Schema:• A set S of relation schemas that belong to the same

database.

• S is the name of the whole database schema

• S = {R1, R2, ..., Rn}

• R1, R2, …, Rn are the names of the individual relation schemas within the database S

• Following slide shows a COMPANY database schema with 6 relation schemas


COMPANY Database Schema


Entity Integrity

• Entity Integrity:• The primary key attributes PK of each relation schema R in S

cannot have null values in any tuple of r(R).• This is because primary key values are used to identify the individual

tuples.

• t[PK] null for any tuple t in r(R)

• If PK has several attributes, null is not allowed in any of these attributes

• Note: Other attributes of R may be constrained to disallow null values, even though they are not members of the primary key.


Referential Integrity

• A constraint involving two relations• The previous constraints involve a single relation.

• Used to specify a relationship among tuples in two relations: • The referencing relation and the referenced relation.


Referential Integrity

• Tuples in the referencing relation R1 have attributes FK (called foreign key attributes) that reference the primary key attributes PK of the referenced relation R2.

• A tuple t1 in R1 is said to reference a tuple t2 in R2 if t1[FK] = t2[PK].

• A referential integrity constraint can be displayed in a relational database schema as a directed arc from R1.FK to R2.


Referential Integrity (or foreign key) Constraint• Statement of the constraint

• The value in the foreign key column (or columns) FK of the referencing relation R1 can be either:

1. A value of an existing primary key value of a corresponding primary key PK in the referenced relation R2, or

2. A null.

• In case (2), the FK in R1 should not be a part of its own primary key.


Displaying a relational database schema and its constraints

• Each relation schema can be displayed as a row of attribute names

• The name of the relation is written above the attribute names

• The primary key attribute (or attributes) will be underlined

• A foreign key (referential integrity) constraints is displayed as a directed arc (arrow) from the foreign key attributes to the referenced table• Can also point the the primary key of the referenced relation for

clarity

• Next slide shows the COMPANY relational schema diagram


Referential Integrity Constraints for COMPANY database


Other Types of Constraints

• Semantic Integrity Constraints:• based on application semantics and cannot be expressed by

the model per se

• Example: “the max. no. of hours per employee for all projects he or she works on is 56 hrs per week”

• A constraint specification language may have to be used to express these

• SQL-99 allows triggers and ASSERTIONS to express for some of these


Populated database state

• Each relation will have many tuples in its current relation state

• The relational database state is a union of all the individual relation states

• Whenever the database is changed, a new state arises

• Basic operations for changing the database:• INSERT a new tuple in a relation

• DELETE an existing tuple from a relation

• MODIFY an attribute of an existing tuple

• Next slide shows an example state for the COMPANY database


Populated database state for COMPANY


Update Operations on Relations

• INSERT a tuple.• DELETE a tuple.• MODIFY a tuple.• Integrity constraints should not be violated by the

update operations.• Several update operations may have to be grouped

together.• Updates may propagate to cause other updates

automatically. This may be necessary to maintain integrity constraints.


Update Operations on Relations

• In case of integrity violation, several actions can be taken:• Cancel the operation that causes the violation (RESTRICT

or REJECT option)

• Perform the operation but inform the user of the violation

• Trigger additional updates so the violation is corrected (CASCADE option, SET NULL option)

• Execute a user-specified error-correction routine


Possible violations for INSERT operation

• INSERT may violate any of the constraints:• Domain constraint:

• if one of the attribute values provided for the new tuple is not of the specified attribute domain

• Key constraint:• if the value of a key attribute in the new tuple already exists in another

tuple in the relation

• Referential integrity:• if a foreign key value in the new tuple references a primary key value

that does not exist in the referenced relation

• Entity integrity:• if the primary key value is null in the new tuple


Possible violations for DELETE operation

• DELETE may violate only referential integrity:• If the primary key value of the tuple being deleted is referenced from

other tuples in the database• Can be remedied by several actions: RESTRICT, CASCADE, SET NULL

(see Chapter 8 for more details)• RESTRICT option: reject the deletion

• CASCADE option: propagate the new primary key value into the foreign keys of the referencing tuples

• SET NULL option: set the foreign keys of the referencing tuples to NULL

• One of the above options must be specified during database design for each foreign key constraint


Possible violations for UPDATE operation

• UPDATE may violate domain constraint and NOT NULL constraint on an attribute being modified

• Any of the other constraints may also be violated, depending on the attribute being updated:• Updating the primary key (PK):

• Similar to a DELETE followed by an INSERT

• Need to specify similar options to DELETE

• Updating a foreign key (FK):• May violate referential integrity

• Updating an ordinary attribute (neither PK nor FK):• Can only violate domain constraints


Summary Part I

• Presented Relational Model Concepts• Definitions

• Characteristics of relations

• Discussed Relational Model Constraints and Relational Database Schemas• Domain constraints’

• Key constraints

• Entity integrity

• Referential integrity

• Described the Relational Update Operations and Dealing with Constraint Violations


2. Data manipulation


Part II Outline

• Relational Algebra• Unary Relational Operations • Relational Algebra Operations From Set Theory• Binary Relational Operations• Additional Relational Operations• Examples of Queries in Relational Algebra

• Relational Calculus• Tuple Relational Calculus• Domain Relational Calculus


Relational Algebra Overview

• Relational algebra is the basic set of operations for the relational model

• These operations enable a user to specify basic retrieval requests (or queries)

• The result of an operation is a new relation, which may have been formed from one or more input relations• This property makes the algebra “closed” (all objects in

relational algebra are relations)


Relational Algebra Overview (continued)

• The algebra operations thus produce new relations• These can be further manipulated using operations of the

same algebra

• A sequence of relational algebra operations forms a relational algebra expression• The result of a relational algebra expression is also a relation

that represents the result of a database query (or retrieval request)


Brief History of Origins of Algebra

• Muhammad ibn Musa al-Khwarizmi (800-847 CE) wrote a book titled al-jabr about arithmetic of variables• Book was translated into Latin.• Its title (al-jabr) gave Algebra its name.

• Al-Khwarizmi called variables “shay”• “Shay” is Arabic for “thing”.• Spanish transliterated “shay” as “xay” (“x” was “sh” in

Spain).• In time this word was abbreviated as x.

• Where does the word Algorithm come from?• Algorithm originates from “al-Khwarizmi"• Reference: PBS (http://www.pbs.org/empires/islam/innoalgebra.html)

http://www.pbs.org/empires/islam/innoalgebra.html

http://www.pbs.org/empires/islam/innoalgebra.html


Relational Algebra Overview

• Relational Algebra consists of several groups of operations• Unary Relational Operations

• SELECT (symbol: (sigma))• PROJECT (symbol: (pi))• RENAME (symbol: (rho))

• Relational Algebra Operations From Set Theory• UNION ( ), INTERSECTION ( ), DIFFERENCE (or MINUS, – )• CARTESIAN PRODUCT ( x )

• Binary Relational Operations• JOIN (several variations of JOIN exist)• DIVISION

• Additional Relational Operations• OUTER JOINS, OUTER UNION• AGGREGATE FUNCTIONS (These compute summary of information:

for example, SUM, COUNT, AVG, MIN, MAX)


Unary Relational Operations: SELECT

• The SELECT operation (denoted by (sigma)) is used to select a subset of the tuples from a relation based on a selection condition.• The selection condition acts as a filter• Keeps only those tuples that satisfy the qualifying condition• Tuples satisfying the condition are selected whereas the other tuples

are discarded (filtered out)• Examples:

• Select the EMPLOYEE tuples whose department number is 4:

DNO = 4 (EMPLOYEE)• Select the employee tuples whose salary is greater than $30,000:

SALARY > 30,000 (EMPLOYEE)


Unary Relational Operations: SELECT

• In general, the select operation is denoted by

<selection condition>(R) where

• the symbol (sigma) is used to denote the select operator

• the selection condition is a Boolean (conditional) expression specified on the attributes of relation R

• tuples that make the condition true are selected• appear in the result of the operation

• tuples that make the condition false are filtered out• discarded from the result of the operation


Unary Relational Operations: SELECT (contd.)

• SELECT Operation Properties• The SELECT operation <selection condition>(R) produces a relation S that has

the same schema (same attributes) as R• SELECT is commutative:

<condition1>( < condition2> (R)) = <condition2> ( < condition1> (R))• Because of commutativity property, a cascade (sequence) of SELECT

operations may be applied in any order: <cond1>(<cond2> (<cond3> (R)) = <cond2> (<cond3> (<cond1> ( R)))

• A cascade of SELECT operations may be replaced by a single selection with a conjunction of all the conditions:

<cond1>(< cond2> (<cond3>(R)) = <cond1> AND < cond2> AND < cond3>(R)))• The number of tuples in the result of a SELECT is less than (or equal

to) the number of tuples in the input relation R


Unary Relational Operations: PROJECT

• PROJECT Operation is denoted by (pi) • This operation keeps certain columns (attributes)

from a relation and discards the other columns.• PROJECT creates a vertical partitioning

• The list of specified columns (attributes) is kept in each tuple• The other attributes in each tuple are discarded

• Example: To list each employee’s first and last name and salary, the following is used:

LNAME, FNAME,SALARY(EMPLOYEE)


Unary Relational Operations: PROJECT (cont.)

• The general form of the project operation is:<attribute list>(R)

(pi) is the symbol used to represent the project operation• <attribute list> is the desired list of attributes from relation

R. • The project operation removes any duplicate tuples

• This is because the result of the project operation must be a set of tuples

• Mathematical sets do not allow duplicate elements.


Unary Relational Operations: PROJECT (contd.)

• PROJECT Operation Properties• The number of tuples in the result of projection <list>(R) is

always less or equal to the number of tuples in R• If the list of attributes includes a key of R, then the number of

tuples in the result of PROJECT is equal to the number of tuples in R

• PROJECT is not commutative <list1> ( <list2> (R) ) = <list1> (R) as long as <list2> contains the

attributes in <list1>


Relational Algebra Expressions

• We may want to apply several relational algebra operations one after the other• Either we can write the operations as a single relational

algebra expression by nesting the operations, or

• We can apply one operation at a time and create intermediate result relations.

• In the latter case, we must give names to the relations that hold the intermediate results.


Single expression versus sequence of relational operations (Example)

• To retrieve the first name, last name, and salary of all employees who work in department number 5, we must apply a select and a project operation

• We can write a single relational algebra expression as follows: FNAME, LNAME, SALARY( DNO=5(EMPLOYEE))

• OR We can explicitly show the sequence of operations, giving a name to each intermediate relation:• DEP5_EMPS DNO=5(EMPLOYEE)

• RESULT FNAME, LNAME, SALARY (DEP5_EMPS)


Unary Relational Operations: RENAME

• The RENAME operator is denoted by (rho)• In some cases, we may want to rename the attributes

of a relation or the relation name or both• Useful when a query requires multiple operations

• Necessary in some cases (see JOIN operation later)


Unary Relational Operations: RENAME (contd.)

• The general RENAME operation can be expressed by any of the following forms: S (B1, B2, …, Bn )(R) changes both:

• the relation name to S, and

• the column (attribute) names to B1, B1, …..Bn

S(R) changes:• the relation name only to S

(B1, B2, …, Bn )(R) changes:• the column (attribute) names only to B1, B1, …..Bn


Unary Relational Operations: RENAME (contd.)

• For convenience, we also use a shorthand for renaming attributes in an intermediate relation:• If we write:

• RESULT FNAME, LNAME, SALARY (DEP5_EMPS)• RESULT will have the same attribute names as DEP5_EMPS

(same attributes as EMPLOYEE)• If we write:

• RESULT (F, M, L, S, B, A, SX, SAL, SU, DNO) FNAME, LNAME, SALARY (DEP5_EMPS)

• The 10 attributes of DEP5_EMPS are renamed to F, M, L, S, B, A, SX, SAL, SU, DNO, respectively


Example of applying multiple operations and RENAME


Relational Algebra Operations fromSet Theory: UNION

• UNION Operation• Binary operation, denoted by • The result of R S, is a relation that includes all tuples

that are either in R or in S or in both R and S• Duplicate tuples are eliminated• The two operand relations R and S must be “type compatible”

(or UNION compatible)• R and S must have same number of attributes• Each pair of corresponding attributes must be type compatible

(have same or compatible domains)


Relational Algebra Operations fromSet Theory: UNION

• Example: • To retrieve the social security numbers of all employees who either work in

department 5 (RESULT1 below) or directly supervise an employee who works in department 5 (RESULT2 below)

• We can use the UNION operation as follows:

DEP5_EMPS DNO=5 (EMPLOYEE)RESULT1 SSN(DEP5_EMPS)

RESULT2(SSN) SUPERSSN(DEP5_EMPS)RESULT RESULT1 RESULT2

• The union operation produces the tuples that are in either RESULT1 or RESULT2 or both


Example of the result of a UNION operation

• UNION Example


Relational Algebra Operations fromSet Theory

• Type Compatibility of operands is required for the binary set operation UNION , (also for INTERSECTION , and SET DIFFERENCE –, see next slides)

• R1(A1, A2, ..., An) and R2(B1, B2, ..., Bn) are type compatible if:• they have the same number of attributes, and

• the domains of corresponding attributes are type compatible (i.e. dom(Ai)=dom(Bi) for i=1, 2, ..., n).

• The resulting relation for R1R2 (also for R1R2, or R1–R2, see next slides) has the same attribute names as the first operand relation R1 (by convention)


Relational Algebra Operations from Set Theory: INTERSECTION

• INTERSECTION is denoted by • The result of the operation R S, is a relation

that includes all tuples that are in both R and S• The attribute names in the result will be the same

as the attribute names in R

• The two operand relations R and S must be “type compatible”


Relational Algebra Operations from Set Theory: SET DIFFERENCE (cont.)

• SET DIFFERENCE (also called MINUS or EXCEPT) is denoted by –

• The result of R – S, is a relation that includes all tuples that are in R but not in S• The attribute names in the result will be the same

as the attribute names in R

• The two operand relations R and S must be “type compatible”


Example to illustrate the result of UNION, INTERSECT, and DIFFERENCE


Some properties of UNION, INTERSECT, and DIFFERENCE

• Notice that both union and intersection are commutative operations; that is• R S = S R, and R S = S R

• Both union and intersection can be treated as n-ary operations applicable to any number of relations as both are associative operations; that is• R (S T) = (R S) T

• (R S) T = R (S T)

• The minus operation is not commutative; that is, in general• R – S ≠ S – R


Relational Algebra Operations from Set Theory: CARTESIAN PRODUCT

• CARTESIAN (or CROSS) PRODUCT Operation• This operation is used to combine tuples from two relations in a

combinatorial fashion.

• Denoted by R(A1, A2, . . ., An) x S(B1, B2, . . ., Bm)

• Result is a relation Q with degree n + m attributes:• Q(A1, A2, . . ., An, B1, B2, . . ., Bm), in that order.

• The resulting relation state has one tuple for each combination of tuples—one from R and one from S.

• Hence, if R has nR tuples (denoted as |R| = nR ), and S has nS tuples, then R x S will have nR * nS tuples.

• The two operands do NOT have to be "type compatible”


Relational Algebra Operations from Set Theory: CARTESIAN PRODUCT (cont.)

• Generally, CROSS PRODUCT is not a meaningful operation• Can become meaningful when followed by other

operations

• Example (not meaningful):• FEMALE_EMPS SEX=’F’(EMPLOYEE)• EMPNAMES FNAME, LNAME, SSN (FEMALE_EMPS)• EMP_DEPENDENTS EMPNAMES x DEPENDENT

• EMP_DEPENDENTS will contain every combination of EMPNAMES and DEPENDENT• whether or not they are actually related


Relational Algebra Operations from Set Theory: CARTESIAN PRODUCT (cont.)

• To keep only combinations where the DEPENDENT is related to the EMPLOYEE, we add a SELECT operation as follows

• Example (meaningful):• FEMALE_EMPS SEX=’F’(EMPLOYEE)• EMPNAMES FNAME, LNAME, SSN (FEMALE_EMPS)• EMP_DEPENDENTS EMPNAMES x DEPENDENT• ACTUAL_DEPS SSN=ESSN(EMP_DEPENDENTS)• RESULT FNAME, LNAME, DEPENDENT_NAME (ACTUAL_DEPS)

• RESULT will now contain the name of female employees and their dependents


Example of applying CARTESIAN PRODUCT


Binary Relational Operations: JOIN

• JOIN Operation (denoted by )• The sequence of CARTESIAN PRODECT followed by SELECT is

used quite commonly to identify and select related tuples from two relations

• A special operation, called JOIN combines this sequence into a single operation

• This operation is very important for any relational database with more than a single relation, because it allows us combine related tuples from various relations

• The general form of a join operation on two relations R(A1, A2, . . ., An) and S(B1, B2, . . ., Bm) is:

R <join condition>S• where R and S can be any relations that result from general relational

algebra expressions.


Binary Relational Operations: JOIN (cont.)

• Example: Suppose that we want to retrieve the name of the manager of each department.• To get the manager’s name, we need to combine each DEPARTMENT

tuple with the EMPLOYEE tuple whose SSN value matches the MGRSSN value in the department tuple.

• We do this by using the join operation.

• DEPT_MGR DEPARTMENT MGRSSN=SSN EMPLOYEE• MGRSSN=SSN is the join condition

• Combines each department record with the employee who manages the department

• The join condition can also be specified as DEPARTMENT.MGRSSN= EMPLOYEE.SSN


Example of applying the JOIN operation


Some properties of JOIN

• Consider the following JOIN operation:• R(A1, A2, . . ., An) S(B1, B2, . . ., Bm)

R.Ai=S.Bj

• Result is a relation Q with degree n + m attributes:• Q(A1, A2, . . ., An, B1, B2, . . ., Bm), in that order.

• The resulting relation state has one tuple for each combination of tuples—r from R and s from S, but only if they satisfy the join condition r[Ai]=s[Bj]

• Hence, if R has nR tuples, and S has nS tuples, then the join result will generally have less than nR * nS tuples.

• Only related tuples (based on the join condition) will appear in the result


Some properties of JOIN

• The general case of JOIN operation is called a Theta-join: R S

theta

• The join condition is called theta• Theta can be any general boolean expression on the

attributes of R and S; for example:• R.Ai<S.Bj AND (R.Ak=S.Bl OR R.Ap<S.Bq)

• Most join conditions involve one or more equality conditions “AND”ed together; for example:• R.Ai=S.Bj AND R.Ak=S.Bl AND R.Ap=S.Bq


Binary Relational Operations: EQUIJOIN

• EQUIJOIN Operation• The most common use of join involves join

conditions with equality comparisons only• Such a join, where the only comparison operator used

is =, is called an EQUIJOIN.• In the result of an EQUIJOIN we always have one or more

pairs of attributes (whose names need not be identical) that have identical values in every tuple.

• The JOIN seen in the previous example was an EQUIJOIN.


Binary Relational Operations: NATURAL JOIN Operation

• NATURAL JOIN Operation • Another variation of JOIN called NATURAL JOIN — denoted by * —

was created to get rid of the second (superfluous) attribute in an EQUIJOIN condition.

• because one of each pair of attributes with identical values is superfluous

• The standard definition of natural join requires that the two join attributes, or each pair of corresponding join attributes, have the same name in both relations

• If this is not the case, a renaming operation is applied first.


Binary Relational Operations NATURAL JOIN (contd.)• Example: To apply a natural join on the DNUMBER attributes of

DEPARTMENT and DEPT_LOCATIONS, it is sufficient to write: • DEPT_LOCS DEPARTMENT * DEPT_LOCATIONS

• Only attribute with the same name is DNUMBER• An implicit join condition is created based on this attribute:

DEPARTMENT.DNUMBER=DEPT_LOCATIONS.DNUMBER

• Another example: Q R(A,B,C,D) * S(C,D,E)• The implicit join condition includes each pair of attributes with the same

name, “AND”ed together:• R.C=S.C AND R.D.S.D

• Result keeps only one attribute of each such pair:• Q(A,B,C,D,E)


Example of NATURAL JOIN operation


Complete Set of Relational Operations

• The set of operations including SELECT , PROJECT , UNION , DIFFERENCE , RENAME , and CARTESIAN PRODUCT X is called a complete set because any other relational algebra expression can be expressed by a combination of these five operations.

• For example: • R S = (R S ) – ((R S) (S R))

• R <join condition>S = <join condition> (R X S)


Binary Relational Operations: DIVISION

• DIVISION Operation• The division operation is applied to two relations • R(Z) S(X), where X subset Z. Let Y = Z - X (and hence Z =

X Y); that is, let Y be the set of attributes of R that are not attributes of S.

• The result of DIVISION is a relation T(Y) that includes a tuple t if tuples tR appear in R with tR [Y] = t, and with

• tR [X] = ts for every tuple ts in S.

• For a tuple t to appear in the result T of the DIVISION, the values in t must appear in R in combination with every tuple in S.


Example of DIVISION


Recap of Relational Algebra Operations


Additional Relational Operations: Aggregate Functions and Grouping

• A type of request that cannot be expressed in the basic relational algebra is to specify mathematical aggregate functions on collections of values from the database.

• Examples of such functions include retrieving the average or total salary of all employees or the total number of employee tuples.• These functions are used in simple statistical queries that summarize

information from the database tuples.• Common functions applied to collections of numeric values

include• SUM, AVERAGE, MAXIMUM, and MINIMUM.

• The COUNT function is used for counting tuples or values.


Aggregate Function Operation

• Use of the Aggregate Functional operation ℱ• ℱMAX Salary (EMPLOYEE) retrieves the maximum salary value from the

EMPLOYEE relation

• ℱMIN Salary (EMPLOYEE) retrieves the minimum Salary value from the EMPLOYEE relation

• ℱSUM Salary (EMPLOYEE) retrieves the sum of the Salary from the EMPLOYEE relation

• ℱCOUNT SSN, AVERAGE Salary (EMPLOYEE) computes the count (number) of employees and their average salary

• Note: count just counts the number of rows, without removing duplicates


Using Grouping with Aggregation

• The previous examples all summarized one or more attributes for a set of tuples• Maximum Salary or Count (number of) Ssn

• Grouping can be combined with Aggregate Functions• Example: For each department, retrieve the DNO, COUNT

SSN, and AVERAGE SALARY• A variation of aggregate operation allows this:ℱ

• Grouping attribute placed to left of symbol• Aggregate functions to right of symbol•

DNO ℱCOUNT SSN, AVERAGE Salary (EMPLOYEE)• Above operation groups employees by DNO (department

number) and computes the count of employees and average salary per department


Additional Relational Operations (cont.)

• Recursive Closure Operations• Another type of operation that, in general, cannot be

specified in the basic original relational algebra is recursive closure.

• This operation is applied to a recursive relationship.

• An example of a recursive operation is to retrieve all SUPERVISEES of an EMPLOYEE e at all levels — that is, all EMPLOYEE e’ directly supervised by e; all employees e’’ directly supervised by each employee e’; all employees e’’’ directly supervised by each employee e’’; and so on.



• Although it is possible to retrieve employees at each level and then take their union, we cannot, in general, specify a query such as “retrieve the supervisees of ‘James Borg’ at all levels” without utilizing a looping mechanism.• The SQL3 standard includes syntax for recursive closure.



•



• The OUTER JOIN Operation• In NATURAL JOIN and EQUIJOIN, tuples without a matching (or

related) tuple are eliminated from the join result• Tuples with null in the join attributes are also eliminated

• This amounts to loss of information.

• A set of operations, called OUTER joins, can be used when we want to keep all the tuples in R, or all those in S, or all those in both relations in the result of the join, regardless of whether or not they have matching tuples in the other relation.



• The left outer join operation keeps every tuple in the first or left relation R in R S; if no matching tuple is found in S, then the attributes of S in the join result are filled or “padded” with null values.

• A similar operation, right outer join, keeps every tuple in the second or right relation S in the result of R S.

• A third operation, full outer join, denoted by keeps all tuples in both the left and the right relations when no matching tuples are found, padding them with null values as needed.



• OUTER UNION Operations• The outer union operation was developed to take the union

of tuples from two relations if the relations are not type compatible.

• This operation will take the union of tuples in two relations R(X, Y) and S(X, Z) that are partially compatible, meaning that only some of their attributes, say X, are type compatible.

• The attributes that are type compatible are represented only once in the result, and those attributes that are not type compatible from either relation are also kept in the result relation T(X, Y, Z).



• Example: An outer union can be applied to two relations whose schemas are STUDENT(Name, SSN, Department, Advisor) and INSTRUCTOR(Name, SSN, Department, Rank).• Tuples from the two relations are matched based on having the same

combination of values of the shared attributes— Name, SSN, Department.• If a student is also an instructor, both Advisor and Rank will have a value;

otherwise, one of these two attributes will be null.• The result relation STUDENT_OR_INSTRUCTOR will have the

following attributes:

STUDENT_OR_INSTRUCTOR (Name, SSN, Department, Advisor, Rank)


Examples of Queries in Relational Algebra

• Q1: Retrieve the name and address of all employees who work for the ‘Research’ department.

RESEARCH_DEPT DNAME=’Research’ (DEPARTMENT)

RESEARCH_EMPS (RESEARCH_DEPT DNUMBER= DNOEMPLOYEEEMPLOYEE)

RESULT FNAME, LNAME, ADDRESS (RESEARCH_EMPS)

• Q6: Retrieve the names of employees who have no dependents.

ALL_EMPS SSN(EMPLOYEE)

EMPS_WITH_DEPS(SSN) ESSN(DEPENDENT)

EMPS_WITHOUT_DEPS (ALL_EMPS - EMPS_WITH_DEPS)

RESULT LNAME, FNAME (EMPS_WITHOUT_DEPS * EMPLOYEE)


Relational Calculus

• A relational calculus expression creates a new relation, which is specified in terms of variables that range over rows of the stored database relations (in tuple calculus) or over columns of the stored relations (in domain calculus).

• In a calculus expression, there is no order of operations to specify how to retrieve the query result—a calculus expression specifies only what information the result should contain. • This is the main distinguishing feature between relational

algebra and relational calculus.


Relational Calculus

• Relational calculus is considered to be a nonprocedural language.

• This differs from relational algebra, where we must write a sequence of operations to specify a retrieval request; hence relational algebra can be considered as a procedural way of stating a query.


Tuple Relational Calculus

• The tuple relational calculus is based on specifying a number of tuple variables.

• Each tuple variable usually ranges over a particular database relation, meaning that the variable may take as its value any individual tuple from that relation.

• A simple tuple relational calculus query is of the form

{t | COND(t)}• where t is a tuple variable and COND (t) is a conditional expression

involving t.

• The result of such a query is the set of all tuples t that satisfy COND (t).


Tuple Relational Calculus

• Example: To find the first and last names of all employees whose salary is above $50,000, we can write the following tuple calculus expression:

{t.FNAME, t.LNAME | EMPLOYEE(t) AND t.SALARY>50000}

• The condition EMPLOYEE(t) specifies that the range relation of tuple variable t is EMPLOYEE.

• The first and last name (PROJECTION FNAME, LNAME) of each EMPLOYEE tuple t that satisfies the condition t.SALARY>50000 (SELECTION SALARY >50000) will be retrieved.


The Existential and Universal Quantifiers

• Two special symbols called quantifiers can appear in formulas; these are the universal quantifier and the existential quantifier

• Informally, a tuple variable t is bound if it is quantified, meaning that it appears in an t or t clause; otherwise, it is free.

• If F is a formula, then so are t)(F) and t)(F), where t is a tuple variable.• The formula t)(F) is true if the formula F evaluates to true for

some (at least one) tuple assigned to free occurrences of t in F; otherwise t)(F) is false.

• The formula t)(F) is true if the formula F evaluates to true for every tuple (in the universe) assigned to free occurrences of t in F; otherwise t)(F) is false.


The Existential and Universal Quantifiers

is called the universal or “for all” quantifier because every tuple in “the universe of” tuples must make F true to make the quantified formula true.

is called the existential or “there exists” quantifier because any tuple that exists in “the universe of” tuples may make F true to make the quantified formula true.


Example Query Using Existential Quantifier

• Retrieve the name and address of all employees who work for the ‘Research’ department. The query can be expressed as :

{t.FNAME, t.LNAME, t.ADDRESS | EMPLOYEE(t) and d) (DEPARTMENT(d) and d.DNAME=‘Research’ and d.DNUMBER=t.DNO) }

• The only free tuple variables in a relational calculus expression should be those that appear to the left of the bar ( | ).• In above query, t is the only free variable; it is then bound successively to each

tuple.• If a tuple satisfies the conditions specified in the query, the attributes

FNAME, LNAME, and ADDRESS are retrieved for each such tuple. • The conditions EMPLOYEE (t) and DEPARTMENT(d) specify the range

relations for t and d. • The condition d.DNAME = ‘Research’ is a selection condition and corresponds

to a SELECT operation in the relational algebra, whereas the condition d.DNUMBER = t.DNO is a JOIN condition.


Example Query Using Universal Quantifier

• Find the names of employees who work on all the projects controlled by department number 5. The query can be:

{e.LNAME, e.FNAME | EMPLOYEE(e) and x)(not(PROJECT(x)) or not(x.DNUM=5)

OR w)(WORKS_ON(w) and w.ESSN=e.SSN and x.PNUMBER=w.PNO))))}• Exclude from the universal quantification all tuples that we are not interested in by

making the condition true for all such tuples.• The first tuples to exclude (by making them evaluate automatically to true) are those that

are not in the relation R of interest. • In query above, using the expression not(PROJECT(x)) inside the universally

quantified formula evaluates to true all tuples x that are not in the PROJECT relation.

• Then we exclude the tuples we are not interested in from R itself. The expression not(x.DNUM=5) evaluates to true all tuples x that are in the project relation but are not controlled by department 5.

• Finally, we specify a condition that must hold on all the remaining tuples in R. w)(WORKS_ON(w) and w.ESSN=e.SSN and x.PNUMBER=w.PNO)


Languages Based on Tuple Relational Calculus

• The language SQL is based on tuple calculus. It uses the basic block structure to express the queries in tuple calculus:• SELECT <list of attributes> • FROM <list of relations> • WHERE <conditions>

• SELECT clause mentions the attributes being projected, the FROM clause mentions the relations needed in the query, and the WHERE clause mentions the selection as well as the join conditions.• SQL syntax is expanded further to accommodate other operations. (See

Chapter 8).


Languages Based on Tuple Relational Calculus

• Another language which is based on tuple calculus is QUEL which actually uses the range variables as in tuple calculus. Its syntax includes:• RANGE OF <variable name> IS <relation name>

• Then it uses• RETRIEVE <list of attributes from range variables>

• WHERE <conditions>

• This language was proposed in the relational DBMS INGRES.


The Domain Relational Calculus

• Another variation of relational calculus called the domain relational calculus, or simply, domain calculus is equivalent to tuple calculus and to relational algebra.

• The language called QBE (Query-By-Example) that is related to domain calculus was developed almost concurrently to SQL at IBM Research, Yorktown Heights, New York. • Domain calculus was thought of as a way to explain what QBE does.

• Domain calculus differs from tuple calculus in the type of variables used in formulas:• Rather than having variables range over tuples, the variables range over single

values from domains of attributes.

• To form a relation of degree n for a query result, we must have n of these domain variables— one for each attribute.


The Domain Relational Calculus

• An expression of the domain calculus is of the form

{ x1, x2, . . ., xn |

COND(x1, x2, . . ., xn, xn+1, xn+2, . . ., xn+m)}

• where x1, x2, . . ., xn, xn+1, xn+2, . . ., xn+m are domain variables that range over domains (of attributes)

• and COND is a condition or formula of the domain relational calculus.


Example Query Using Domain Calculus

• Retrieve the birthdate and address of the employee whose name is ‘John B. Smith’.

• Query : {uv | (q) (r) (s) (t) (w) (x) (y) (z)

(EMPLOYEE(qrstuvwxyz) and q=’John’ and r=’B’ and s=’Smith’)}• Ten variables for the employee relation are needed, one to range over the

domain of each attribute in order. • Of the ten variables q, r, s, . . ., z, only u and v are free.

• Specify the requested attributes, BDATE and ADDRESS, by the free domain variables u for BDATE and v for ADDRESS.

• Specify the condition for selecting a tuple following the bar ( | )—• namely, that the sequence of values assigned to the variables qrstuvwxyz be a

tuple of the employee relation and that the values for q (FNAME), r (MINIT), and s (LNAME) be ‘John’, ‘B’, and ‘Smith’, respectively.


Part II Summary

• Relational Algebra• Unary Relational Operations • Relational Algebra Operations From Set Theory• Binary Relational Operations• Additional Relational Operations• Examples of Queries in Relational Algebra

• Relational Calculus• Tuple Relational Calculus• Domain Relational Calculus


3. Implementation

SQL 99 Standard


SQL Standard

• SQL: Structured Query Language• Pronounced “S-Q-L” or “sequel”• SQL was specified in 1970s • SQL was enhanced substantially in 1989 and 1992• A new standard called SQL3 added object-oriented

features• A subset of SQL3 standard, now known as SQL-99

has been approved


SQL

• DDL statements• CREATE, ALTER, DROP

• SCHEMA, TABLE, VIEW, CONSTRAINT, TRIGGER

• Update• INSERT, DELETE, UPDATE

• DML• SELECT


Rules for NULL’s

• When we operate on a NULL and another value (including another NULL) using +, –, etc., the result is NULL

• Aggregate functions ignore NULL, except COUNT(*) (since it counts rows)

• When we compare a NULL with another value (including another NULL) using =, >, etc., the result is UNKNOWN.

• WHERE and HAVING clauses only select rows for output if the condition evaluates to TRUE. UNKNOWN is insufficient


Unfortunate consequences

• SELECT AVG(GPA) FROM Student;

• SELECT SUM(GPA)/COUNT(*) FROM Student;

• Not equivalent

• Although AVG(GPA)= SUM(GPA)/COUNT(GPA), still

• SELECT * FROM Student;

• SELECT * FROM Student WHERE GPA = GPA;

• Not equivalent

• Be careful: NULL breaks many equivalences


Another problem

• Example: Who has NULL GPA values?

• SELECT * FROM Student WHERE GPA = NULL;• Does not work; never returns anything

• (SELECT * FROM Student)

EXCEPT ALL

(SELECT * FROM Student WHERE GPA = GPA)

• Works, but ugly

• Introduced built-in predicates IS NULL and IS NOT NULL

• SELECT * FROM Student WHERE GPA IS NULL;


E F Codd’s Relational Model of Data (1)

• Relational model and normal form• Data independence

• Data Dependencies• Ordering dependencies

• Indexing dependencies

• Access path dependencies



• Relational model and normal form• Relational view of data

• Relation

• Properties of a relation

• Each row represents an n-tuple of R.

• The ordering of rows is immaterial.

• All rows are distinct.

• The ordering of columns is significant--it corresponds to the ordering S1, S2, - . - , Sn of the domains on which R is defined

• The significance of each column is partially conveyed by labeling it with the name of the corresponding domain.



• Relational view of data• Normal form

• Normalization

• Some linguistic aspects• Expressible, named, and stored relations

• Named set is the collection of all those relations that the community of users can identify by means of a simple name (or identifier)

• Expressible set is the total collection of relations that can be designated by expressions in the data language

• Named set is a subset of the expressible set --- usually a very small subset

• Stored relations



• Redundancy and consistency• Operations on relations

1. Permutations

2. Projection

3. Join

4. Composition

5. Restriction

• Redundancy• Strong redundancy

• Weak redundancy

• Consistency



• Homework assignment Read the paper carefully and develop answers to the following questions:1. What is data dependency? Explain three forms of

data redundancies discussed in the paper.2. Explain each of the five operators with an example.3. What is redundancy? How strong redundancy is

different than weak redundancy?4. Discuss the idea of consistency as explained in the

paper.

• Due: Saturday 8 March 2008 (Typed printed copy in class)


Summary

• Relational data model1. Data Management

• Informal definitions• Formal definitions• Integrity constraints• Update operations

2. Data Manipulation• Relational algebra• Relational calculus

• Tuple calculus• Domain calculus

3. Implementation• SQL

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