Fundamentals of database system - Relational data model and relational database constraints

Fundamentals of Database systems
Ministry of Higher Education
Bamyan University
Computer Science Department
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Presented by : Mustafa Kamel Mohammadi
Email : bamian.cs@gmail.com
Relational data model and relational database constraints
Fundamentals of database system 6th edition

Learning objective
 In his chapter you will learn
 Relational data model concepts
 Relational data model constraints
 Characteristics of relation
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Relational Model Concepts
The Relational model represents the database as a collection of relations.
Each relation resembles a table of values
Each row in the table represents a collection of related data values. A row
represents a
 Real-world entity
 Relationship
Table name and column names are used to help to interpret the meaning of the
values in each row.
In the formal relational model terminology
 A row is called a tuple
 A column header is called an attribute
 The table is called a relation
 The data type describing the types of values that can appear in each column is represented
by a domain of possible values
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Cont.
Domain
A domain D is a set of atomic values.
By atomic we mean that each value in the domain is indivisible
Method of specifying a domain is to specify a data type
 Usa_phone_numbers. The set of ten-digit phone numbers valid in the United States.
 Social_security_numbers. The set of valid nine-digit Social Security numbers.
 (This is a unique identifier assigned to each person in the United States for employment, tax, and
benefits purposes.)
 Names. The set of character strings that represent names of persons.
The preceding are called logical definitions of domains.
A domain is thus given a name, data type, and format
 ISBN = xxx-xxxx-xxxx-xxxx , x can contain (0-9, A-Z)
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Cont.
Relation schema
A relation schema R, denoted by R(A1, A2, ...,An) is made up of a relation name R
and a list of attributes, A1, A2, ..., An
Each attribute Ai is identified by dom (Ai)
The degree of a relation is the number of attributes of its relation schema
A relation of degree seven
 STUDENT(Name, Ssn, Home_phone, Address, Office_phone, Age, Gpa)
Using the data type of each attribute, the definition is sometimes written as:
 STUDENT(Name: string, Ssn: string, Home_phone: string, Address: string,Office_phone:
string, Age: integer, Gpa: real)
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
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Cont.
Relation state
Denoted by r(R), is a set of n-tuples r = {t1, t2, ..., tn}.
Denoted by lower case letter (r)
Each tuple (t) is an ordered list of n values t=<v1,v2, ..., vn>
each value vi, 1 ≤ i ≤ n, is an element of dom (Ai) or is a special NULL value
It is possible for several attributes to have the same domain. The attribute names
indicate different roles
 USA_phone_numbers plays the role of Home_phone
 USA_phone_numbers plays the role of office_phone
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Characteristics of Relations
Ordering of Tuples in a Relation
 Logically because relation is defined as a set of tuples. Mathematically, elements of a set
have no order among them.
 Physically there always is an order among the records. This ordering indicates first,
second, ith, and last records in the file
 When we display a relation as a table, the rows are displayed in a certain order by
different attributes.
Values and NULLs in the Tuples
 Each value in a tuple is an atomic value
 Composite and multivalued attributes are not allowed
 Null is used to represent values that are
 value unknown
 value exists but is not available
 Attribute does not apply
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Cont.
Interpretation (Meaning) of a Relation
The relation schema can be interpreted as a declaration.
Each tuple in the relation can then be interpreted as a fact or a particular
instance of the assertion
 Some relations may represent facts about entities
 Whereas other relations may represent facts about relationships.
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Relational Model Constraints
There are generally many restrictions or constraints on the actual values in a
database state
Constraints on databases can generally be divided into three main categories:
 Constraints that are inherent in the data model. We call these inherent model-based
constraints or implicit constraints
 Constraints that can be directly expressed in schemas of the data model, typically by
specifying them in the DDL ,We call these schema-based constraints or explicit constraints.
 Constraints that cannot be directly expressed in the schemas of the data model, and hence
must be enforced by the application programs. We call these application-based or semantic
constraints or business rules.
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Explicit or Schema Based constraints
Domain constraints
Specify that within each tuple, the value of each attribute A must be an atomic
value.
Key Constraints
Means that no two tuples can have the same combination of values for all their
attributes.
Subsets of attributes of a relation schema R with the property that no two tuples
in any relation state r of R is called a super key.
A super key can have redundant attributes so removing those attributes makes
the concept of key.
 Key is a minimal super key that is, a super key from which we cannot remove any attributes
and still have the uniqueness constraint
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Cont.
 A key is also a super key but not vice versa
 Example: {Ssn, Name, Age} is a super key for R (STUDENT) but not a key, because removing Name
and Age still leaves us with a super key.
 The value of a key attribute can be used to identify uniquely each tuple in the relation.
 A relation schema may have more than one key. In this case, each of the keys is called a candidate
key
 For example, the CAR relation in Figure 3.4 has two candidate keys: License_number and
Engine_serial_number
 Primary key of the relation is the candidate key whose values are used to identify tuples in the
relation. Primary keys are underlined in schema diagram.
 The choice of one to become the primary key is arbitrary.
 The other candidate keys are designated as unique keys.
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Cont.
Null value constraints
Another constraint on attributes specifies whether NULL values are or are not
permitted.
NULL
 Can have or have not value
NOT NULL
 Must have value, it can not be empty
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Relational Databases and Relational Database Schema
A relational database usually contains many relations, with tuples in relations that
are related in various ways.
A relational database schema S is a set of relation schemas S = {R1, R2, ..., Rm }.
A relational database state DB of relational schema S is a set of relation states DB
= {r1, r2, ..., rm} such that each ri is a state of Ri
Example:
 COMPANY = {EMPLOYEE, DEPARTMENT,DEPT_LOCATIONS, PROJECT, WORKS_ON,
DEPENDENT}
When we refer to a relational database, we implicitly include both its schema and
its current state.
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Cont.
Attributes that represent different concepts may have the same name in different
relations
For example
 we could have used the attribute name Name for both PROJECT and DEPARTMENT.
The same real-world concept is used in different roles (meanings) in the same
relation
 Ssn and super_ssn …. Must have different names
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Integrity constraint
Integrity constraint states that no primary key value can be NULL
 because the primary key value is used to identify individual tuples in a relation. Having NULL
values for the primary key implies that we cannot identify some tuples
Key constraints and entity integrity constraints are specified on individual
relations.
Referential integrity constraint is specified between two relations and is used to
maintain the consistency and relationship among tuples in the two relations.
the referential integrity constraint states that a tuple in one relation that refers to
another relation must refer to an existing tuple in that relation
 Dno of EMPLOYEE gives the department number for which each employee works,hence, its
value in every EMPLOYEE tuple must match the Dnumber value of some tuple in the
DEPARTMENT relation
Referential integrity constraint is maintained by foreign keys.
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20
Referential integrity constraints

Foreign key
Specify a referential integrity constraint between the two relation schemas R1
and R2.
A set of attributes FK in relation schema R1 is a foreign key of R1 that references
relation R2 if it satisfies the following rules.
 The attributes in FK have the same domain(s) as the primary key attributes PK of R2.
 A value of FK in a tuple t1 of the current state r1(R1) either occurs as a value of PK for some
tuple t2 in the current state r2(R2) or is NULL. In the former case, we have t1[FK] = t2[PK]
R1 is called the referencing relation and R2 is the referenced relation.
A foreign key could have NULL values
 when doesn’t related to other relation
 when it is not being used as primary key
a foreign key can refer to its own relation
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Other types of constraints
Semantic integrity constraints
 the salary of an employee should not exceed the salary of the employee’s supervisor
 maximum number of hours an employee can work on all projects per week is 56.
 specified and enforced within the application programs that update the database
General-purpose constraint specification language.
 Use Mechanisms called triggers and assertions
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Operations and Constraint violation
The operations of the relational model can be categorized into retrievals and
updates
 retrievals operations
 database modification or update operations
three basic operations that can change the states of relations in the database:
Insert, Delete, and Update
Whenever these operations are applied, the integrity constraints specified on the
relational database schema should not be violated
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Insert operation
The Insert operation provides a list of attribute values for a new tuple t that is to
be inserted into a relation R
insert can violate any of the four types of constraints discussed
 Domain constraints can be violated if an attribute value is given that does not appear in the
corresponding domain
 Key constraints can be violated if a key value in the new tuple t already exists in another tuple
in the relation r(R)
 Entity integrity can be violated if any part of the primary key of the new tuple t is NULL
 Referential integrity can be violated if the value of any foreign key in t refers to a tuple that
does not exist in the referenced relation
If an insertion violates one or more constraints, the default option is to reject the
insertion
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Delete operation
The Delete operation can violate only referential integrity.
This occurs if the tuple being deleted is referenced by foreign keys from other
tuples in the database.
Several options are available if a deletion operation causes a violation
 restrict rejects the deletion
 Cascade attempts to cascade (or propagate) the deletion by deleting tuples that reference
the tuple that is being deleted
 set null
 set default
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The Update operation
 The Update (or Modify) operation is used to change the values of one or more attributes in a
tuple (or tuples) of some relation R.
 Updating an attribute that is neither part of a primary key nor of a foreign key usually causes no
problems.
 check to confirm that the new value is of the correct data type and domain
 Modifying a primary key value is similar to deleting one tuple and inserting another in its place
 If a foreign key attribute is modified, the DBMS must make sure that the new value refers to an
existing tuple in the referenced relation
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The Transaction Concept
A database application program running against a relational database typically
executes one or more transactions
A transaction is an executing program that includes some database operations,
such as
 reading from the database
 applying insertions, deletions, or updates to the database.
At the end of the transaction, it must leave the database in a valid or consistent
state that satisfies all the constraints specified on the database schema
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Fundamentals of database system - Relational data model and relational database constraints

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