Key in DatabaseManagementSystem_engineering.pptx

Primary key
• A primary is a column or set of columns in a
table that uniquely identifies tuples (rows) in
that table.

Points to Note regarding Primary Key
• The value of primary key should be unique for
each row of the table.
• The attribute(s) that is marked as primary key is
not allowed to have null values.
• Primary keys are not necessarily to be a single
attribute (column). It can be a set of more than
one attributes (columns).
• For example {Stu_Id, Stu_Name} collectively can
identify the tuple unique in the above table, but
we do not choose it as primary key
because Stu_Id alone is enough to uniquely
identifies rows in a table and we always go for
minimal set.

Example-1
Stu_Name alone cannot be a primary key as more than one students can have same name.
Stu_Age alone cannot be a primary key as more than one students can have same age.
Stu_Id alone is a primary key as each student has a unique id that can identify the
student record in the table.

Example-2
Customer_ID alone cannot be a primary key as a single customer can place more than
one order. Thus more than one rows of same Customer_ID value. Example in the above
table customer id 1011has placed two orders with product _ID 9023 and 9111.
Product_ID alone cannot be a primary key as more than one customers can place a
order for the same product .Thus more than one rows with same product id.Example
in the above table customer id 1011 & 1122 placed an order for the same product
(product id 9023).

….
• Order Quantity alone cannot be a primary key
as more than one customers can place the
order for the same quantity.
• Since none of the attributes alone were able
to become a primary key, So
• {Customer_ID, Product_ID} together can
identify the rows uniquely in the table so this
set is the primary key for this table.

Super key
• A super key is a set of one of more columns
(attributes) to uniquely identify rows in a
table.
• Let’s take an example.

..
• The above table has following super keys.
• All of the following sets of super key are able to
uniquely identify a row of the employee table.
• {Emp_SSN}
• {Emp_Number}
• {Emp_SSN, Emp_Number}
• {Emp_SSN, Emp_Name}
• {Emp_SSN, Emp_Number, Emp_Name}
• {Emp_Number, Emp_Name}

Candidate Key
• Candidate keys are selected from the set of
super keys.
• The only thing we take care while selecting
candidate key is ,It should not have any
redundant attribute.
• That’s the reason they are also termed as
minimal super key.

Example
Super Keys
{Emp_SSN}
{Emp_Number}
{Emp_SSN, Emp_Number}
{Emp_SSN, Emp_Name}
{Emp_SSN, Emp_Number, Emp_Name}
{Emp_Number, Emp_Name}
Candidate keys(without redundant
attributes)
{Emp_SSN}
{Emp_Number}
{Emp_SSN, Emp_Number}
{Emp_SSN, Emp_Name}
{Emp_SSN, Emp_Number, Emp_Name}
{Emp_Number, Emp_Name}

…
• The following two set of super keys are chosen
from the above sets as there are no redundant
attributes in these sets.
• {Emp_SSN}
• {Emp_Number}
• Only these two sets are candidate keys as all
other sets are having redundant attributes
that are not necessary for unique
identification.

Primary key
• A Primary key is selected from a set of candidate
keys.
• This is done by database admin or database
designer.
• Note: While choosing a set of attributes for a
primary key, we always choose the minimal set
that has minimum number of attributes.
• For example, if there are two sets that can
identify row in table, the set that has minimum
number of attributes should be chosen as
primary key.
• We can say that
either {Emp_SSN} or {Emp_Number} can be
chosen as a primary key for the table Employee.

Alternate key
• Out of all candidate keys, only one gets
selected as primary key, remaining keys are
known as alternate or secondary keys.

…
• There are two candidate keys in the above
table:
{Emp_Id}
{Emp_Number}
• DBA (Database administrator) can choose any
of the above key as primary key.
• Lets say Emp_Id is chosen as primary key.
• Since we have selected Emp_Id as primary key,
the remaining key Emp_Number would be
called alternative or secondary key.

Foregin key
• Foreign keys are the columns of a table that
points to the primary key of another table.
They act as a cross-reference between tables.

…
• Student table
In the below example the Stu_Id column in Course_enrollment table is a
foreign key as it points to the primary key of the Student table.

Composite key
• A key that has more than one attributes is
known as composite key. It is also known as
compound key.
• Any key such as super key, primary
key, candidate key etc. can be called
composite key if it has more than one
attributes.

Key in DatabaseManagementSystem_engineering.pptx

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