Relational database

1. DATABASE
MANAGEMENT SYSTEMS
Unit / Topic: II /I

2. Relational Model
• Primary data model for commercial data processing
applications.
• Consists of relational databases.
• Advantages:
Simplicity
Easier for accessing data

3. Structure of Relational Databases
• Relational database  collection of tables with unique
names
• Tables  called as relations
• Rows of the table  records or tuples
• Columns of the table  attributes or fields

4. Contd..
• Set of permitted values of the attributes  domain
values
• Null value  special value that signifies that the
value is unknown or does not exist

5. Example

6. Database Schema and Instance
• Database schema:
Logical design or overall design of the database
Does not change frequently
Eg: university (instructor, student)
• Database instance:
Snapshot of the data in the database at a given
instant in time.
Changes frequently

7. Relation Schema and Instance
Relation schema:
• Consists of a list of attributes and their corresponding
domains.
• Similar to a variable in programming language.
• Does not change frequently
• Eg: instructor = (ID, name, dept_name, salary)
Relation instance:
• Set of domain values in a relation at a particular point of
time or current values
• Similar to value of a variable in programming language
• Changes frequently

8. Example
Schema
Instance

9. Keys
Types of keys:
Super key
Candidate key
Primary key
Foreign key

10. Primary Key
• Main reference key of the table
• One of the candidate keys that is most appropriate
to uniquely identify a record in a table.

11. Candidate Key
• Subset of super key
• Minimal super key
• Single field or least combination of fields that
uniquely identifies each record in the table

12. Super Key
• A super key is a set of attributes of any combination
that uniquely identifies a record within a table.
• Given table:
STUDENT{StudentId, firstName, lastName,
courseId}
• Possible superkeys are:
o {StudentId}
o {StudentId, firstName}
o {firstName, lastName}
o (StudentId, firstName, lastName, courseId}

13. Foreign key
• Used to relate two tables
• Primary key of one table (parent table) acts as a key
for another table (child table) in order to provide
relationship between these two tables
• That non-primary key attribute is said to be the
foreign key for the parent table
• Child table is called as the referencing relation and
the parent table is called as the referenced relation

14. Example

15. Referential Integrity Constraints
• Set of constraints applied to foreign key which
prevents entering a row in child table (where you
have foreign key) for which you don't have any
corresponding row in parent table.
• Referential Integrity prevents your table from
having incorrect or incomplete relationship.

16. Schema Diagrams
• Pictorial representation of database schemas
• A database schema along with primary key and
foreign key dependencies can be depicted by
schema diagrams

17. Schema Diagram for the University Database

18. Formal Query Languages
Relational Algebra

19. Introduction
• Query  retrieval of information
• Query language  language that is used for
information retrieval
• Formal query languages  based upon which SQL
is developed
• Types:
Relational Algebra
Relational Calculus
Tuple Relational Calculus
Domain Relational Calculus

20. Relational Algebra
• Procedural query language
• Consists of a set of operations that take one or two
relations as input and produce a new single relation as
their output

21. Fundamental Operations
• Select
• Project
• Union
• Set difference
• Cartesian product
• Rename
Note: select, project, rename – called as ‘unary’
operators and the remaining are ‘binary’ operators

22. Select Operation
• The select operation display tuples that satisfy a
given predicate (condition).
• We use the lowercase Greek letter sigma to
denote selection.
• It produces a “horizontal” subset.
• Syntax: sC(R)
– where C is a selection condition(=, <, >, =<, >=,
<>)
– and R is the relation over which the selection
takes place

23. Example of Select
Student
Find all students whose id is above 300.
Query: s sid > 300(Student)
Resulting Relation:
sid name addr
123 Fred 3 Oxford
345 John 6 Hope Rd.
567 Ann 5 Garden
sid name addr
345 John 6 Hope Rd.
567 Ann 5 Garden

24. Project Operation
• The project operation display attributes that
satisfy a given predicate (condition).
• We use the lowercase Greek letter pi to
denote selection.
• It produces a “vertical” subset.
• Eliminates duplicated values.
• Syntax: ПA(R)
– where A is a set of attributes of R
– and R is the relation over which the project
takes place

25. Example of Project
Enrollment
Display all sid values
Query: Пsid(Enrollment)
sid name addr
123 Fred 3 Oxford
345 John 6 Hope Rd.
567 Ann 5 Garden
sid
123
345
567

27. SELECTION & PROJECTION Example
Id Name Address Hobby
1123 John 123 Main stamps
1123 John 123 Main coins
5556 Mary 7 Lake Dr hiking
9876 Bart 5 Pine St stamps
Id Name Address Hobby
1123 John 123 Main stamps
9876 Bart 5 Pine St stamps
σ Hobby=‘stamps’(Person)
Person
∏Name, Hobby(Person)
Name Hobby
John stamps
John coins
Mary Hiking
Bart stamps

28. 28
Examples
•  Id>3000 OR Hobby=‘hiking’ (Person)
•  Id>3000 AND Id <3999 (Person)
•  NOT(Hobby=‘hiking’) (Person)
•  Hobby‘hiking’ (Person)
•  Id (Person)
•  Hobby (Person)

29. Relational Algebra Expressions

30. Union Operation
• Creates a relation that contains all the values in both
the relations.
• Two relations are union compatible if:
– Both have same number of columns
– Names of attributes are the same in both
• Represented using the symbol ‘∪’.
• Eliminate duplicates
• Syntax: S1 ∪ S2
– where S1, S2 are separate relations

31. Example

32. The Set-Difference Operation
• Allows us to find tuples that are in one relation but
are not in another.
• Denoted by the symbol ‘−’.
• Syntax: S1 – S2
• where S1, S2 are separate relations

33. Example

34. Example

35. SUMMARY
INTRODUCTION TO RELATIONAL MODEL
• Structure of Relational Databases
o Tables
o Records or tuples
o Attributes or fields
o Domain values
o Null values
• Difference b/w database schema and database instance
• Difference b/w relation schema and relation instance
• Keys
o Super key
o Candidate key
o Primary key
o Foreign key
• Referential integrity constraint
• Fundamental database operations