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数据库管理系统:ch02 Relational Model.ppt
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数据库管理系统:ch02 Relational Model.ppt
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©Silberschatz, Korth and Sudarshan2.2Database System Concepts - 5
th
Edition, June 15, 2005
Chapter 2: Relational Model
Chapter 2: Relational Model
Structure of Relational Databases
Fundamental Relational-Algebra-Operations
Additional Relational-Algebra-Operations
Extended Relational-Algebra-Operations
Null Values
Modification of the Database
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©Silberschatz, Korth and Sudarshan2.3Database System Concepts - 5
th
Edition, June 15, 2005
Example of a Relation
Example of a Relation
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©Silberschatz, Korth and Sudarshan2.4Database System Concepts - 5
th
Edition, June 15, 2005
Basic Structure
Basic Structure
Formally, given sets D
1
, D
2
, …. D
n
a relation r is a subset of
D
1
x D
2
x … x D
n
Thus, a relation is a set of n-tuples (a
1
, a
2
, …, a
n
) where each a
i
D
i
Example: If
customer_name = {Jones, Smith, Curry, Lindsay}
customer_street = {Main, North, Park}
customer_city = {Harrison, Rye, Pittsfield}
Then r = { (Jones, Main, Harrison),
(Smith, North, Rye),
(Curry, North, Rye),
(Lindsay, Park, Pittsfield) }
is a relation over
customer_name x customer_street x customer_city
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©Silberschatz, Korth and Sudarshan2.5Database System Concepts - 5
th
Edition, June 15, 2005
Attribute Types
Attribute Types
Each attribute of a relation has a name
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
Note: multivalued attribute values are not atomic
Note: composite attribute values are not atomic
The special value null is a member of every domain
The null value causes complications in the definition of many operations
We shall ignore the effect of null values in our main presentation and
consider their effect later
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