chapter 5 relational model concepts

Chapter 5 Relational Model Concepts

Dr. Bernard Chen Ph.D.University of Central Arkansas

Fall 2008

Relational Model Concepts 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

Outline

Relational Model Concepts Domains, Attributes, and Relations Characteristics of Relations

Relational Model Constrains and Relational Database Schemas

Update Operations, Transactions, and Dealing with Constrain Violations

Relational Model Concepts The relational model represents the

database as a collection of relations

Each relation resembles a table of values

When a relation is thought of as a table of values, each row in the table represents a collection of related data values

Formal Terminology A row is called a tuple A column header is called an

attribute The table is called relation

Domain A Domain D is a set of atomic values.

Atomic means that each value in the domain is indivisible as far as the relational model is concerned

It means that if we separate an atomic value, the value itself become meaningless, for example:

SSN Local_phone_number Names Employee_ages

Relation Schema Relation Schema R, denoted by R(A1, A2,…, An), is

made up of relation name R and a list of attributes A1, A2, …,An

Each attribute Ai is the name of a role played by some domain D in the relation schema R.

D is called the domain of Ai and is denoted by dom(Ai)

R is called the name of the relation

The degree of a relation is the number of attributes n of its relation schema

Formal Definitions 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

Formal Definitions 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)

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

Outline




Ordering

Ordering of Tuples is a Relation a relation is defined as a set of tuples. Mathematically, elements of a set have

NO order among them The ordering indicates first, second, ith,

and last records in the file Hence, the following two relations are

identical

Identical Relations

Values in the Tuples Each value in a tuple is an atomic value

Hence, composite and multi-valued attributes are not allowed

This model is sometimes called the flat relational model

Much of the theory behind the relational model was developed with this assumption, which is called first normal form assumption

Null in tuples

An important concept is that if NULL values, which are used to represent the values of attributes that may be unknown or may not apply to a tuple

Relational Model Notation

An attribute A can be qualified with the relation name R to which it belongs by using the dot notation R.A

For example, STUDENT.Name or STUDENT.Age

Relational Model NotationWe 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 Both t[Ai, Aj, Ak] or t.(Ai, Aj, Ak) refers to a list of

attributes from R

For example: consider a tuple t=< “Barbara Benson”, “533-69-1238”, “839-8461”, “7384 Fontana Lane”, NULL, 19, 3.25> from the STUDENT relation in Figure 5.1

We have t.name=< “Barbara Benson”,> t. (Ssn, Gpa, Age) = <“533-69-

1238”,3.25,19>

Outline




Domain Constrains Each attribute A must be an atomic

value from the dom(A)

The data types associated with domains typically include standard numeric data type for integers, real numbers, Characters, Booleans, fix-length strings, time, date, money or some special data types

Key Constrains A relation is defined as a set of tuples By definition, all elements of a set are

distinct This means that no two tuples can have

the same combination of values for all their attributes

Superkey: a set of attributes that no two distinct tuples in any state r of R have the same value

Every relation has at least one default superkey – the set of all its attributes

Key Constrains

A superkey can have redundant attributes, so a more useful concept is that of a KEY which has no redundancy

Key satisfied two constrains: Two distinct tuple in any state of the

relation cannot have identical values for the attributes in the key

It is a minimal superkey

Key Constrains For example, consider STUDENT

relation The attribute set {SSN} is a key of

STUDENT because no two student can have the same value for SSN

Any set of attributes that includes SSN – for example {SSN, Name, Age} – is a superkey

Key Constrains In general, a relation schema may have

more than one key, in this case, each of the key is called a candidate key

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.

Key Constrains

If a relation has several candidate keys, one is chosen arbitrarily to be the primary key.

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

Key Constrains and Constrains on NULL values

Another constraint on attributes specifies whether NULL value are or are not permitted

For example, if every STUDENT tuple must have a valid, non-NULL value for the Name attribute, then Name of STUDENT is constrained to be NOT NULL

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 Constraint Referential Integrity Constraint is specified

between two relations and is used to maintain the consistency among tuples in the two relations

Informally define the constrain: a tuple in one relation must refer to an existing tuple in that relation

For example, the Dno in EMPLOYEE gives the department number for which each employee works, this number must match the Dnumber value in DEPARTMENT

Referential Integrity Constraint 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].

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

Outline




Modification and Updates In this section, we concentrate on the

database Updates and Modification

There are threee basic operation: Insert, Delete and Modify Insert is used to insert a new tuple or tuples

in a relation Delete is used to delete tuples Update (or Modify) is used to change the

values of some attributes

Modification and Updates Insert: insert new element with specify

all related attributes

Delete: delete an element by giving Relation name and key of the tuple

Modify: modify a value by giving a relation name, Key of the target tuple and attribute to modify

Possible violations for each 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

Insert Example Insert <‘Cecilia’, ‘F’, ‘Kolonsky’, NULL, ‘1960-

04-05’, ‘6357 Windy lane,Kate,TX’, F, 28000, NULL, 4> into EMPLOYEE

Insert < ‘Cecilia’, ‘F’, ‘Kolonsky’, 999887777, ‘1960-04-05’, ‘6357 Windy lane,Kate,TX’, F, 28000, NULL, 4 >

Cecilia’, ‘F’, ‘Kolonsky’, 667788999, ‘1960-04-05’, ‘6357 Windy lane,Kate,TX’, F, 28000, NULL, 7>

Possible violations for each 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

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

Delete Example Delete the EMPLOYEE tuple with

Ssn=‘99988777’

Delete the EMPLOYEE tuple with Ssn=‘333445555’

Delete the EORKS_ON tuple eith Essn=‘999887777’ and Pno=10

Possible violations for each 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

Update Example Update the salary of EMPLOYEE tuple with

Ssn=‘999887777’ to 2800

Update the Dno of the EMPLOYEE tuple with Ssn=‘999887777’ to 1

Update the Dno of the EMPLOYEE tuple with Ssn=‘999887777’ to 7

Update the Ssn of the EMPLOYEE tuple with Ssn=‘999887777’ to ‘987654321’

chapter 5 relational model concepts

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