Different Types of Keys used in Database systems are explained.

1. Ms. B.K.Ugale(bharati_ugale@rediffmail.com) 1
Types of Keys in Database System

2. Types of Keys
 Super Key
 Candidate Key
 Primary Key
 Alternate Key
 Composite Key
 Foreign Key
 Unique Key
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3. Super Key
 An attribute or set of attributes that are required to identifies a
tuple in a relation.
 A Super Key may contain additional attributes that are not
require for unique identification.
 Unique Key-Student_ID
 Super Key:
 {Student_ ID, Student_Name}
 {Studenr_ID, Student_Address}
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4. Candidate Key
 Candidate Keys are minimal set of Super Key
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5. Primary Key
 Primary Key is a candidate key chosen by database administrator for
unique identification.
 No Two rows can have the primary key same values.
 Every row must have primary key value
 The primary key value can not be null
 Value in primary key can not be modified.
 Primary Key: PAN No.
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6. Alternate Key
• Candidate keys other than primary key are called as a alternate
key.
• PK: PAN No.
• Alternate Key: Voter Id No, Passport No
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7. Composite Key
• Composite key is the one having combination of more than one attribute
to uniquely determine the records/tuples in a table.
• The only condition that is necessary before choosing composite key is,
the attributes which are responsible for forming the composite key must
not be a key individually.
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8. Foreign Key
• A foreign key is nothing but an attribute that is commonly linked between two
relation using that same attribute and that attribute must be primary key from one
relation.
• Both the relations/tables must contain the same attribute.
• It can be referred as referential integrity
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9. Unique Key
 Unique key is same as primary key with the difference being
the existence of Null value.
 Unique Key: MemberID
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10. Ms. B.K.Ugale(bharati_ugale@rediffmail.com) 10
All Keys

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