Slide 5 keys

KEYS
Prepared by Visakh V, Assistant
Professor,Dept. of CSE, LBSITW

KEYS
 Used to establish and identify relation between
tables
 Each record with in a table can be uniquely
identified bye combination of one more fields in a
table
 Help to enforce integrity and identify relationship

Different types of keys
1. Super Key
2. Candidate Key
3. Primary Key
4. Foreign Key
5. Secondary / Alternate key
6. Simple Key
7. Compound Key
8. Composite Key

Super Key
 A Super key is any combination of fields
within a table that uniquely identifies each
record within that table.
 Superset of candidate key

Candidate Key
 A subset of a super key.
 A candidate key is a single field or the least
combination of fields that uniquely identifies
each record in the table.
 The least combination of fields distinguishes
a candidate key from a super key. (ie.Minimal
Set of Super Key)
Every table must have at least one candidate
key but at the same time can have several.

In order to be eligible for a candidate key it
must pass certain criteria
I. It must contain unique values
II. Must not contain null values
III.Contain minimum number of fields to
ensure uniqueness
IV. Must uniquely identify each record in a
table
Note: Once your candidate keys have been identified
you can now select one to be your primary key

Primary Key
A primary key is a candidate key that is most
appropriate to be the main reference key for the
table.
It is the primary key of reference for the table
It is used throughout the database to help
establish relationships with other tables.

 Primary key must contain
• Unique values
• Must never be null
• Uniquely identify each record in the
table.
• Note : Primary keys are mandatory for
every table each record must have a value
for its primary key.
• Note: When choosing a primary key from
the pool of candidate keys always choose a
single simple key over a composite key.

Foreign Key
 Foreign key is a column(s) that references a
column(s) of a table and it can be same table also
 A foreign key is generally a Unique key from one
table that appears as a field in another where the
first table has a relationship to the second.
 In other words, if we had a table A with a
Unique key X that linked to a table B where X was a
field in B, then X would be a foreign key in B.

This relationship ensures
• Records cannot be inserted into a detail table if
corresponding record in the master table do not exist.
• Record of a master table cannot be deleted if
corresponding records in the detail table actually exist

Parent
(or)
Master Table
Child
(or) Detail
table

Parent must be UNIQUE OR PRIMARY KEY
Child may have DUPLICATE/NULL unless it is
specified
 Constraint specified on child not on parent
Parent record can delete only if no child
record exist
Parent cannot modify if child record exist

Secondary / Alternate Key
 A table may have one or more choices for
the primary key. Collectively these are
known as candidate keys.
 One is selected as the primary key. Those
not selected are known as secondary keys
or alternative keys.

Simple Key
 A simple key consists of a single field to uniquely
identify a record.
In addition the field in itself cannot be broken
down into other fields

Compound Key
A compound key consists of more than one
field to uniquely identify a record.
 Each attribute that makes up the compound
key is a simple key in its own right.

Composite Key
 A composite key consists of more than one
field to uniquely identify a record.
 This differs from a compound key in that one
or more of the attributes, which make up the
key, are not simple keys in their own right

Slide 5 keys

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Slide 5 keys