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Transcript of intro to relational models
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8/17/2019 intro to relational models
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Database System Concepts, 6th Ed.
©Silberschatz, Korth and Sudarshan
See www.db-book.com for conditions on re-use
Chapter 2: Intro to Relational Model
http://www.db-book.com/http://www.db-book.com/http://www.db-book.com/http://www.db-book.com/
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©Silberschatz, Korth and Sudarshan2.2Database System Concepts - 6th Edition
Example of a Relation
attributes(or columns)
tuples
(or rows)
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©Silberschatz, Korth and Sudarshan2.3Database System Concepts - 6th Edition
Attribute Types
The set of allowed values for each attribute is called the domain
of the attribute
Attribute values are (normally) required to be atomic; that is,
indivisible
The special value null is a member of every domain
The null value causes complications in the definition of many
operations
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©Silberschatz, Korth and Sudarshan2.4Database System Concepts - 6th Edition
Relation Schema and Instance
A1, A2, …, An are attributes
R = ( A1, A2, …, An ) is a relation schema
Example:
instructor = (ID, name, dept_name, salary)
Formally, given sets D1, D2, …. Dn a relation r is a subset of
D1 x D2 x … x Dn
Thus, a relation is a set of n-tuples (a1, a2, …, an) where each ai ∈ Di
The current values (relation instance) of a relation are specified by
a table
An element t of r is a tuple, represented by a row in a table
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©Silberschatz, Korth and Sudarshan2.6Database System Concepts - 6th Edition
Database
A database consists of multiple relations
Information about an enterprise is broken up into parts
instructor
student
advisor
Bad design:univ (instructor -ID, name, dept_name, salary, student_Id, ..)
results in
repetition of information (e.g., two students have the same instructor)
the need for null values (e.g., represent an student with no advisor) Normalization theory (Chapter 7) deals with how to design “good”
relational schemas
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Keys
Let K ⊆ R
K is a superkey of R if values for K are sufficient to identify a uniquetuple of each possible relation r(R)
Example: {ID} and {ID,name} are both superkeys of instructor.
Superkey K is a candidate key if K is minimal
Example: {ID} is a candidate key for Instructor
One of the candidate keys is selected to be the primary key.
which one?
Foreign key constraint: Value in one relation must appear in another
Referencing relation Referenced relation
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Schema Diagram for University Database
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Relational Query Languages
Procedural vs.non-procedural, or declarative
“Pure” languages:
Relational algebra
Tuple relational calculus
Domain relational calculus
Relational operators
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Selection of tuples
Relation r
Select tuples with A=B
and D > 5
σ A=B and D > 5 (r)
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Selection of Columns (Attributes)
Relation r :
Select A and C
Projection
Π A, C (r)
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Joining two relations – Cartesian Product
Relations r, s:
r x s:
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Union of two relations
Relations r, s:
r ∪ s:
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Set difference of two relations
Relations r , s:
r – s:
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Set Intersection of two relations
Relation r, s:
r ∩ s
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Joining two relations – Natural Join
Let r and s be relations on schemas R and S respectively.
Then, the “natural join” of relations R and S is a relation on
schema R ∪ S obtained as follows:
Consider each pair of tuples tr from r and ts from s.
If tr and ts have the same value on each of the attributes
in R ∩ S, add a tuple t to the result, where
t has the same value as tr on r
t has the same value as ts on s
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Natural Join Example
Relations r, s:
Natural Join
r s
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Figure in-2.1
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Database System Concepts, 6th Ed.
©Silberschatz, Korth and Sudarshan
See www.db-book.com for conditions on re-use
End of Chapter 2
http://www.db-book.com/http://www.db-book.com/http://www.db-book.com/http://www.db-book.com/
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Figure 2.01
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Figure 2.02
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Figure 2.03
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©Silberschatz, Korth and Sudarshan2.23Database System Concepts - 6th Edition
Figure 2.04
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©Silberschatz, Korth and Sudarshan2.24Database System Concepts - 6th Edition
Figure 2.05
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©Silberschatz, Korth and Sudarshan2.25Database System Concepts - 6th Edition
Figure 2.06
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Figure 2.07
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©Silberschatz, Korth and Sudarshan2.27Database System Concepts - 6th Edition
Figure 2.10
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©Silberschatz, Korth and Sudarshan2.28Database System Concepts - 6th Edition
Figure 2.11
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©Silberschatz, Korth and Sudarshan2.29Database System Concepts - 6th Edition
Figure 2.12
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Figure 2.13