Chapter 4

Rational data model

Chapter 5Rational data model

Chapter 4Rational

data model

Contents

Relational Model Concepts Relational Model Constraints and Relational

Database Schemas

The things you should know…

The relational Model of Data is based on the concept of a Relation.

The basic principle of relational database is proposed by E.F.Codd in 1970.

The first RDBMS production is System R The most popular RDBMS

DB2, Oracle, Ingres, Sybase, Informix,…

Informal Definitions

RELATION: A table of values A relation may be thought of as a set of rows.

Each row represents a fact that corresponds to a real-world entity or relationship.

Each row has a value of an item or set of items that uniquely identifies that row in the table.

Each column typically is called by its column name or column header or attribute name.

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

In the STUDENT table, SSN is the key

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

Example - Figure 5.1

Basic of the Relational Model

The relational model gives us a single way to represent data: as a two- dimensional table called a relation.

Attributes ( 属性 ) Attributes of a relation serve us as names for the col

umns of the relation. Usually, the attributes describes the meaning of enti

tes in the column below.

Schemas （模式） The name of a relation and the set of attributes for a

relation is called the schema for that relation. We show the schema for the relation with the relati

on name followed by a parenthesized list of its attributes.

Tuples （元组） The rows of a relation, other than the header row cont

aining the attributes, are called tuple. For example:

Relations, however, are sets of tuples, and it is impossible for a tuple to appear more than once in a given relation.

Domains （域） The relational model requires that each component of e

ach tuple be atomic; that is, it must be of some elementary type such as integers or string.

Each attributes of a relation has particular elementary type, thus domain is decided.

Formal Definitions

A domain has a logical definition.

Example: “USA_phone_numbers” are the set of 10 digit phone numbers valid in the U.S.

A domain also has a data-type or a format defined for it. For example, the USA_phone_numbers may have a forma

t: (ddd)-ddd-dddd where each d is a decimal digit.

Dates have various formats such as month, date, year or yyyy-mm-dd, or dd mm,yyyy etc.

Equivalent Representations of a Relation The attributes of the relation can be reordered without

change the relation.

Relation Instance （关系实例） A relation about movies is not static; rather, relatio

ns change over time. It is not common for the schema of a relation to cha

nge. We shall call a set of tuples for a given relation an i

nstance of that relation.

DEFINITION SUMMARY

Informal Terms Formal Terms

Table Relation

Column Attribute/Domain

Row Tuple

Values in a column Domain

Table Definition Schema of a Relation

Populated Table Extension

An Example of Relation InstanceRelation: Person (Name, Address, Telephone)

Relation Instance:

Name Address Telephone

Bob 123 Main St 555-1234

Bob 128 Main St 555-1235

Pat 123 Main St 555-1235

Harry 456 Main St 555-2221

Sally 456 Main St 555-2221

Sally 456 Main St 555-2223

Pat 12 State St 555-1235

More …

Relation (Instance) = a set of tuples Database = collection of relations Relation schema = relation name + attributes

Example: Movies (title, year, length, fileType)

Database schema = a set of all relation schemas Movies(Title, Year, Length, FileType) Star(Name, Age) Studio(StudioName, Addr)

Name Addr Tel N1 A1 T1 N2 A2 T2 N3 A3 T3 N4 T4 N5 T5 T6 T7

Name Addr Tel N1 A1 T1 N1 A1 T2 N1 A1 T3 . . .

N1 A1 T7 N1 A2 T1 N1 A3 T1 N2 A1 T1

Tuple

Domain

Component

Attribute

Characteristics of Relations

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

gh they appear to be in the tabular form.

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 th

e values in t=<v1, v2, ..., vn> not to be ordered .

 Values in a tuple: All values are considered atomic (indivisible). A special null value is used to represent values that are unknown or inapplicable to certain tuples.

Chapter 5-22

Key

Example: The STUDENT relation schema:STUDENT(stu_num, stu_name, gender, major,

Year)has two keys Key1 = {stu_num}, Key2 = {stu_na

me}candidate keys:just keysIf a relation has several candidate keys, one is

chosen arbitrarily to be the primary key. The primary key attributes are underlined.

Relational Integrity Constraints

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

There are three main types of constraints in the relational model: Entity integrity constraints Referential integrity constraints User-define Constraints Users define the constrains themselves

Another implicit constraint is the domain constraint Every value in a tuple must be from the domain of its attribu

te (or it could be null, if allowed for that attribute)

Chapter 5-24

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)and have different values. This is because primary key values are used to identify the individual tuples.

Entity integrity

i.e ： student （ stu_num ， name ， gender ， age,major_num ）

　　 course （ course_num ， curse_name ， teacher ）

　　 select_course （ stu_num ， course_num ， grades ）

Chapter 5-26

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.

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].

Chapter 5-27

Referential Integrity Constraint

Statement of the constraintThe value in the foreign key column (or

columns) FK of the the referencing relation R1 can be either: (1) a value of an existing primary key value of the 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.

i.e １：　 student （ stu_num ， name ， gender ， age,major_num ）

　　 major （ major_num ， major_name ）

Foreign key Referen

cing relationReferenc

ed relation

student 　　　　　　 majorMajor_num

Chapter 5-29

Other Types of Constraints

Constraints:- based on application semantics and cannot be

expressed by the model.- E.g., “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 allow for some of these

Chapter 5-30

Chapter 5-31

5.6

Chapter 5-32

5.7