key
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KeyDBMS
Prepared By: Bikash SapkotaDeerwalk Ins. Of Technology
Kathmandu, Nepal
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Bikash S
apkota
Introduction
Keys(Name clearly define)
A key part of a relational database and a vital part of the structure of a table. They ensure each record within a table can be uniquely identified by one or a combination of fields within the table. They help enforce integrity and help identify the relationship between tables.
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Keys
Super Key
Candidate Key
Primary Key
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Super Key
A Super key is any combination of fields within a table that uniquely identifies each record within that table.
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BookBook ID Name Author
B1 Xyz A1
B2 ABC A1
B3 XYZ A2
B4 PQR A3
B5 RST A1
B6 ABC A3
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Candidate Key
Candidate key is a subset of super key
A candidate key is a single field or the least combination of fields that uniquely identifies each record in table
The least combination of fields distinguishes a candidate key from a super key.
Every table must have at least one candidate
Properties Unique
Not null
Contains minimum number of fields to ensure uniqueness
Must uniquely identify each record in the table
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Book ID Name Author
B1 XYZ A!
B2 ABC A1
B3 XYZ A2
B4 PQR A3
B5 RST A1
B6 ABC A3
Name BookId Author Name Author
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Primary Key
It is a candidate key
Uniquely identify a specific instance of an entity
Primary key cannot contain any Null value because we cannot uniquely identify multiple Null values.
Properties Stable
Minimal
Definitive
Accessible
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Book ID Name Author
B1 Xyz A!
B2 ABC A1
B3 XYZ A2
B4 PQR A3
B5 RST A1
B6 ABC A3
Book ID Name Author
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CREATE TABLE Persons(
P_Id int NOT NULL PRIMARY KEY,LastName varchar(255) NOT NULL,FirstName varchar(255),Address varchar(255),City varchar(255)
);
CREATE TABLE Person
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
);
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Foreign Key
Foreign key is a field in a relational table that matches the primary key column of another table.
It can be used to link two tables together
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CREATE TABLE supplier
( supplier_id numeric(10) not null,
supplier_name varchar2(50) not null,
contact_name varchar2(50),
CONSTRAINT supplier_pk PRIMARY KEY (supplier_id)
);
CREATE TABLE products
( product_id numeric(10) not null,
supplier_id numeric(10) not null,
CONSTRAINT fk_supplier
FOREIGN KEY (supplier_id)
REFERENCES supplier(supplier_id)
);
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Remaining
Alternate Key
Remaining candidate key which are not selected as primary key
Composite Key
Primary key made up of more than one attribute