The Relational Model CIS 218. Entity A Person, Place, Thing or Transaction Something the user wants...
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Transcript of The Relational Model CIS 218. Entity A Person, Place, Thing or Transaction Something the user wants...
The Relational Model
CIS 218
Entity
• A Person, Place, Thing or Transaction
• Something the user wants to track
Attribute
• A Property that describes an entity
• Also called a Field
Title
Author
Number of Pages
Subject
???
Attribute
• A Property that describes an entity
Identifier
• An attribute that names, or identifies, an entity instance
• May or may not be unique
Ludwig
Primary Key
• A unique identifier
Relation
• A relation is a two-dimensional table that has specific characteristics
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Characteristics of a Relation
• Consists of columns and rows• Each row contains data about one entity• Each columns contains data about one attribute
of the entity• All entries in a column are of the same kind• Each cell of the table holds a single value• Each column has a unique name• No two rows may be identical
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Is this a Relation?
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EmployeeNumber Phone LastName
100 335-6421
454-9744
Abernathy
101 215-7789 Cadley
104 610-9850 Copley
107 299-9090 Jackson
Is this a Relation?
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EmployeeNumber Phone LastName
100 335-6421 Abernathy
101 215-7789 Cadley
104 610-9850 Copley
100 335-6421 Abernathy
107 299-9090 Jackson
Is this a Relation?
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EmployeeNumber
Phone Name Name
100 335-6421 Robin Abernathy
101 215-7789 Jessica Cadley
104 610-9850 Roger Copley
105 222-8873 Gurpreet Kaur
107 299-9090 Jazmyne Jackson
Is this a Relation?
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EmployeeNumber
Phone Name Number
100 335-6421 Robin 123
101 215-7789 Jessica $27.45
104 610-9850 Roger Fourteen
105 222-8873 Gurpreet 37
107 299-9090 Jazmyne 5-06-65
Is this a Relation?
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EmployeeNumber FirstName LastName
100 Mary Abernathy
101 Jerry Cadley
104 Alex Copley
107 Megan Jackson
A Composite Key
• A primary key made up of two or more attributes (columns)
• For a key to be unique, often it must become a composite key– Example: How would I look up a grade?
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A Candidate Key
• A candidate to become the primary key• A unique identifier
– SID, SSN
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A Surrogate Key
• A unique, numeric value that is added to a relation to serve as the Primary Key
• Has no meaning to users • Usually hidden in forms, queries and reports• In Access, uses an AutoNumber
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Relationships
• Associations between entities
Relationships Between Tables
• A table may be related to other tables– An Employee is assigned to a Car– A Student takes a Class
• To relate tables, create a foreign key– Also called a linking column
• A foreign key is a primary key from one table placed into another table
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Foreign Key Example
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Project
ProjID
ProjName
MgrID
Manager
MgrID
MgrName
Foreign Key
Foreign Key Example
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Department
DeptID
DeptName
Location
Employee
EmpID
DeptID
EmpName
Foreign Key
Foreign Key Example
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Song
SongID
SongName
Time
Artist
ArtistID
ArtistName
Foreign KeyArtistID
Referential Integrity
• Every foreign key value must match a value of an existing primary key
• Therefore, the primary key value must exist before the foreign key value is entered
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Referential Integrity
Each value of the Foreign Key OwnerID in PET
must exist in
The values of the Primary Key OwnerID in OWNER
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Foreign Keys inMicrosoft Access
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