cse 301: database systems ch2
TRANSCRIPT
Database System Concepts, 6th Ed.
©Silberschatz, Korth and SudarshanSee www.db-book.com for conditions on re-use
Chapter 2: Chapter 2: Introduction to Relational ModelIntroduction to Relational Model
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Relational ModelRelational Model
The relational data model is based on a collection of tables each of which is assigned a unique name.
The user of the database system may query these tables, insert new tuples, delete tuples, and update (modify) tuples. There are several languages for expressing these operations.
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Example of a RelationExample of a Relation
attributes(or columns)
tuples(or rows)
Relation:
A Table
Tuple:
A Row
Attributes:
A Column header
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Attribute TypesAttribute 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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Relation Schema and InstanceRelation Schema and Instance
The schema of a relation refers to its logical design. The schema of a relation includes its attributes, and optionally the types of the attributes and constraints on the relation such as primary and foreign key constraints.
Example: instructor(id, name, dept_name, salary)
An instance of the relation refers to its contents at a point in time.
The current values (relation instance) of a relation are specified by the rowa of a table.
An element t of r is a tuple, represented by a row in a table.
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Relations are UnorderedRelations are Unordered
Order of tuples is irrelevant (tuples may be stored in an arbitrary order)
Example: instructor relation with unordered tuples
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DatabaseDatabase
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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KeysKeys
Let K R
K is a superkey of R if values for K are sufficient to identify a unique tuple 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 DatabaseSchema Diagram for University Database
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Relational Query LanguagesRelational Query Languages
Procedural vs.non-procedural, or declarative
Procedural: the user instructs the system to perform a sequence of operations on the database to compute the desired result.
Non-procedural: the user describes the desired information without
giving a specific procedure for obtaining that information.
“Pure” and formal languages:
Relational algebra
Tuple relational calculus
Domain relational calculus
Relational operators
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Selection of tuplesSelection 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)Selection of Columns (Attributes)
Relation r:
Select A and C
Projection
Π A, C (r)
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Joining two relations – Cartesian ProductJoining two relations – Cartesian Product
Relations r, s:
r x s:
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Union of two relationsUnion of two relations
Relations r, s:
r s:
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Set difference of two relationsSet difference of two relations
Relations r, s:
r – s:
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Set Intersection of two relationsSet Intersection of two relations
Relation r, s:
r s
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Joining two relations – Natural JoinJoining 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 ExampleNatural Join Example
Relations r, s:
Natural Join
r s
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Figure in-2.1Figure in-2.1
Database System Concepts, 6th Ed.
©Silberschatz, Korth and SudarshanSee www.db-book.com for conditions on re-use
End of Chapter 2End of Chapter 2
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Figure 2.01Figure 2.01
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Figure 2.02Figure 2.02
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Figure 2.03Figure 2.03
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Figure 2.04Figure 2.04
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Figure 2.05Figure 2.05
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Figure 2.06Figure 2.06
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Figure 2.07Figure 2.07
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Figure 2.10Figure 2.10
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Figure 2.11Figure 2.11
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Figure 2.12Figure 2.12
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Figure 2.13Figure 2.13