Relational Model and Terminology - Introduction to Databases

1. Lecture 2
Relational Model and Terminology

2. Objectives
• In this lecture, you will learn:
– Some relational model terminologies
– The difference between the relation schema and
relational database schema
– The properties of a relation
– Relational keys
– Relational intergrity
– Views and purpose of views in relational database
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3. Relational Model Terminology
• An entity is a person, place, event, or
thing/Object for which we intend to collect
data
• Examples:
– University -- Students, Faculty Members, Courses
– Airline -- Pilots, Aircraft, Routes, Suppliers
• A relation is a table with columns and rows.
– Only applies to logical structure of the database,
not the physical structure.

4. Relational Model Terminology
• Attribute is a named column of a relation.
• Example:
– Student -- Student Number, Name...etc
• Domain is the set of allowable values for one
or more attributes.
Attribute Domain Name Meaning Domain
Definition
Stu_id StudentID ID of a student Character:
Size 10

5. Relational Model Terminology
• Tuple is a row of a relation.
• Degree is the number of attributes in a relation.
• Cardinality is the number of tuples in a relation.
• Relational Database
– A collection of normalized relations with distinct
relation names.

6. 6
Example of Attribute Domain

7. 7
Entity and Attribute

8. Alternative Terminology
Attribute
Entity Occurrence
Entity Class
Conceptual data modeling notation
Student File
Id No. Name Faculty Major
1100 Ali FIT SE
1200 Mohamed FIT MIS
1300 Aisha FCM MM
1400 Hawwa FCM MM
1500 Hussain FOE CE

9. Alternative Terminology
Attribute
Field
Entity Occurrence
Record
Entity Class
File
Conceptual data modeling notation
File Processing Notation
Student File
Id No. Name Faculty Major
1100 Ali FIT SE
1200 Mohamed FIT MIS
1300 Aisha FCM MM
1400 Hawwa FCM MM
1500 Hussain FOE CE

10. Alternative Terminology
Attribute
Field
Column
Entity Occurrence
Record
Row
Entity Class
File
Table
Conceptual data modeling notation
File Processing Notation
Relational Table Notation
Student File
Id No. Name Faculty Major
1100 Ali FIT SE
1200 Mohamed FIT MIS
1300 Aisha FCM MM
1400 Hawwa FCM MM
1500 Hussain FOE CE

11. Database Relations
• Relation schema
• Named relation defined by a set of attribute and domain
name pairs.
• Relational database schema
• Set of relation schemas, each with a distinct name.

12. Relation and Relational Schema
• A relation schema is the name and the list of
attributes of a relation
• Example:
– Grade (studentId, assignmentId, points, dateSubmitted)
Relational Schema

13. Properties of Relation
• Relation name is distinct from all other relation names
in relational schema.
• Each cell of relation contains exactly one atomic (single)
value.
• Each attribute has a distinct name.
• Values of an attribute are all from the same domain.
• Each tuple is distinct; there are no duplicate tuples.
• Order of attributes has no significance.
• Order of tuples has no significance, theoretically.

14. Relational Keys
• Super key
– An attribute, or a set of attributes, that uniquely identifies a
tuple within a relation.
• Candidate Key
– Super key (K) such that no proper subset is a super key within
the relation.
• Candidate key, K, for a relation R has two properties
– Uniqueness - in each tuple of R, values of K uniquely identify
that tuple.
– Irreducibility - no proper subset of K has the uniqueness
property

15. Relational Key
• Super key
– Uniquely identifies each entity
• STU_NUM
• STU_NUM, STU_LNAME
• STU_NUM, STU_LNAME, STU_INIT
• Candidate key
– Minimal super key
• STU_NUM
• STU_LNAME, STU_FNAME, STU_INIT, STU_PHONE
• When a key consists of more than one attribute, it is
called a composite key

16. Example
• Which attributes of this table can be used as a candidate
key?
• BranchNo and PostalCode can be used as candidate keys.
• City attribute cannot be used as a candidate key, because it
has duplicate values.
BranchNo Street City PostalCode
B005 22 Deer Rd London SW1 4EH
B007 16 Argyll St Aberdeen AB2 3SU
B003 163 Main St Glasgow G11 9QX
B004 32 Manse Rd Bristol BS99 1NZ
B003 56 Clover Dr London NW10 6EU

17. Relational Key
• Primary Key
– Candidate key selected to identify tuples uniquely within
relation.
• Alternate Keys
– Candidate keys that are not selected to be primary key.
• Foreign Key
– Attribute, or set of attributes, within one relation that matches
candidate key of some (possibly same) relation

18. Relational Integrity
• Null
– Represents value for an attribute that is currently unknown or
not applicable for tuple.
– Deals with incomplete or exceptional data.
– Represents the absence of a value and is not the same as zero
or spaces, which are values.
– Example: if a student is absent for the final exam. The final
exam value of that student should be null.

19. Relational Integrity
• Entity Integrity
– In a base relation, no attribute of a primary key can be null.
• Primary key (BranchNo)
• Cannot insert a tuple in this relation if BranchNo is null.
BranchNo Street City PostalCode
B005 22 Deer Rd London SW1 4EH
B007 16 Argyll St Aberdeen AB2 3SU
B003 163 Main St Glasgow G11 9QX
B004 32 Manse Rd Bristol BS99 1NZ
B006 56 Clover Dr London NW10 6EU

20. Relational Integrity
• Entity Integrity
– In a base relation, no attribute of a primary key can
be null.
• Composite primary key (ClientNo, PropertyNo)
• Cannot insert a tuple in this relation if either ClientNo
is null, PropertyNo is null or both ClientNo and
PropertyNo are null.
ClientNo PropertyNo ViewDate Comment
CR56 PA14 24 May 01 Too small
CR76 PG4 20 April 01 Too remote
CR56 PG4 16 May 01
CR62 PA14 14 May 01 No dining room
CR56 PG36 28 April 01

21. Relational Integrity
• Referential Integrity
– If foreign key exists in a relation, either foreign key value
must match a candidate key value of some tuple in its home
relation or foreign key value must be wholly null.

22. 22
Relational Integrity
BranchNo Street City PostalCode
B005 22 Deer Rd London SW1 4EH
B007 16 Argyll St Aberdeen AB2 3SU
B003 163 Main St Glasgow G11 9QX
B004 32 Manse Rd Bristol BS99 1NZ
B006 56 Clover Dr London NW10 6EU
staffNo fName lName Position Sex Salary BranchNo
SL21 Jhon White Manager M 30000 B005
SG37 Ann Beech Assistant F 12000 B003
SG14 David Ford Supervisor M 18000 B003
SA9 Mary Howe Assistant F 9000 B007
SG5 Susan Brand Manager F 24000 B003
SL41 Julie Lee Assistant F 9000 B005

23. Referential Integrity: Example
• BranchNo in the staff relation is a foreign key
targeting the BranchNo attribute in the Branch
relation.
• Not possible to create a staff record with non
existing BrancNo (example with B025)
• But it is possible to create a staff record with a
null branch number to cater for new staff who
has not assigned any branch office.
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24. Referential Integrity: Example
• Consider the situation where we have two tables:
Employees and Managers. The Employees table
has a foreign key attribute entitled ManagedBy
which points to the record for that employee’s
manager in the Managers table.
– We may not add a record to the Employees table
unless the ManagedBy attribute points to a valid
record in the Managers table.
– If the primary key for a record in the Managers table
changes, all corresponding records in the Employees
table must be modified using a cascading update.
– If a record in the Managers table is deleted, all
corresponding records in the Employees table must be
deleted using a cascading delete.
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25. Views
• Base Relation
– Named relation corresponding to an entity in conceptual schema,
whose tuples are physically stored in database.
• View
– Dynamic result of one or more relational operations operating on
base relations to produce another relation.
– A view is a virtual relation that does not necessarily exists in a
database but can be produced upon request.
– Contents of a view are defined as a query on one or more base
relations.
– Views are dynamic, meaning that changes made to base relations
that affect view attributes are immediately reflected in the view.

26. Views: Examples
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Student
StudentID Student Name DOB Mark1 Mark2 Mark3
00001 Mohamed 2/2/1985 56 85 74
00002 Ahmed 3/3/1990 85 45 25
00003 Aisha 4/4/1992 75 95 35
00004 Hawwa 6/6/1989 56 54 87
Query1
StudentID Mark1 Mark2 Mark3
00001 56 85 74
00002 85 45 25
00003 75 95 35
00004 56 54 87
Query2
StudentID Student Name Mark1
00001 Mohamed 56
00002 Ahmed 85
00003 Aisha 75
00004 Hawwa 56

27. Purpose of Views
• Provides powerful and flexible security mechanism by hiding
parts of database from certain users.
• Permits users to access data in a customized way, so that same
data can be seen by different users in different ways, at the
same time.
• Can simplify complex operations on base relations ( Example:
join).
• A view should be designed to support external model:
– Some members of the staff should be able to see staff tuples
without the salary attribute.
– Attributes may be renamed or the order of attributes changed.
– Some members of the staff should see only a specific records.