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KEYS in Relational Model

The document discusses different types of keys in the relational model: Candidate keys are attributes that uniquely identify tuples. There can be more than one candidate key and they can be simple or composite. Super keys are attributes that uniquely identify tuples and include candidate keys plus additional attributes. The primary key is the chosen candidate key, while alternate keys are the other candidate keys. Foreign keys are attributes whose values must match the primary key of another relation and enforce referential integrity between relations.

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60 views2 pages

KEYS in Relational Model

The document discusses different types of keys in the relational model: Candidate keys are attributes that uniquely identify tuples. There can be more than one candidate key and they can be simple or composite. Super keys are attributes that uniquely identify tuples and include candidate keys plus additional attributes. The primary key is the chosen candidate key, while alternate keys are the other candidate keys. Foreign keys are attributes whose values must match the primary key of another relation and enforce referential integrity between relations.

Uploaded by

MariaMichaelVisuwasam
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as DOC, PDF, TXT or read online on Scribd
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Different Types of Keys in Relational Model

Candidate Key: The minimal set of attribute which can uniquely identify a tuple is known
as candidate key. For Example, STUD_NO in STUDENT relation.
 The value of Candidate Key is unique and non-null for every tuple.
 There can be more than one candidate key in a relation. For Example,
STUD_NO as well as STUD_PHONE both are candidate keys for relation
STUDENT.
 The candidate key can be simple (having only one attribute) or composite as
well. For Example, {STUD_NO, COURSE_NO} is a composite candidate key for
relation STUDENT_COURSE.

Note – In Sql Server a unique constraint that has a nullable column, allows the value
‘null‘ in that column only once. That’s why STUD_PHONE attribute as candidate here,
but can not be ‘null’ values in primary key attribute.

Super Key: The set of attributes which can uniquely identify a tuple is known as Super
Key. For Example, STUD_NO, (STUD_NO, STUD_NAME) etc.

 Adding zero or more attributes to candidate key generates super key.


 A candidate key is a super key but vice versa is not true.

Primary Key: There can be more than one candidate key in a relation out of which one
can be chosen as primary key. For Example, STUD_NO as well as STUD_PHONE both
are candidate keys for relation STUDENT but STUD_NO can be chosen as primary key
(only one out of many candidate keys).
Alternate Key: The candidate key other than primary key is called as alternate key. For
Example, STUD_NO as well as STUD_PHONE both are candidate keys for relation
STUDENT but STUD_PHONE will be alternate key (only one out of many candidate
keys).

Foreign Key: If an attribute can only take the values which are present as values of
some other attribute, it will be foreign key to the attribute to which it refers. The relation
which is being referenced is called referenced relation and corresponding attribute is
called referenced attribute and the relation which refers to referenced relation is called
referencing relation and corresponding attribute is called referencing attribute.
Referenced attribute of referencing attribute should be primary key. For Example,
STUD_NO in STUDENT_COURSE is a foreign key to STUD_NO in STUDENT relation.

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