Keys in DBMS-

Keys in DBMS-

Different Types Of Keys in DBMS-

There are following 10 important keys in DBMS-

1. Super key
2. Candidate key
3. Primary key
4. Alternate key
5. Foreign key
6. Partial key
7. Composite key
8. Unique key
9. Surrogate key
10. Secondary key

1. Super Key-

• A super key is a set of attributes that can identify each tuple uniquely in the given relation.
• A super key is not restricted to have any specific number of attributes.
• Thus, a super key may consist of any number of attributes.

Example-

Consider the following Student schema-

Student ( roll , name , sex , age , address , class , section )

Given below are the examples of super keys since each set can uniquely identify each student in the Student table-

• ( roll , name , sex , age , address , class , section )
• ( class , section , roll )
• (class , section , roll , sex )
• ( name , address )

NOTE-

All the attributes in a super key are definitely sufficient to identify each tuple uniquely in the given relation but all of them may not be necessary.

2. Candidate Key-  A set of minimal attribute(s) that can identify each tuple uniquely in the given relation is called as a candidate key.

Example-

Consider the following Student schema-

Student ( roll , name , sex , age , address , class , section )

Given below are the examples of candidate keys since each set consists of minimal attributes required to identify each student uniquely in the Student table-

• ( class , section , roll )
• ( name , address )

NOTES-

• All the attributes in a candidate key are sufficient as well as necessary to identify each tuple uniquely.
• Removing any attribute from the candidate key fails in identifying each tuple uniquely.
• The value of candidate key must always be unique.
• The value of candidate key can never be NULL.
• It is possible to have multiple candidate keys in a relation.
• Those attributes which appears in some candidate key are called as prime attributes.

3. Primary Key-

A primary key is a candidate key that the database designer selects while designing the database.

OR

Candidate key that the database designer implements is called as a primary key.

NOTES-

• The value of primary key can never be NULL.
• The value of primary key must always be unique.
• The values of primary key can never be changed i.e. no updation is possible.
• The value of primary key must be assigned when inserting a record.
• A relation is allowed  to have only one primary key.

Remember-

4. Alternate Key-

Candidate keys that are left unimplemented or unused after implementing the primary key are called as alternate keys.

OR

Unimplemented candidate keys are called as alternate keys.

5. Foreign Key-

• An attribute ‘X’ is called as a foreign key to some other attribute ‘Y’ when its values are dependent on the values of attribute ‘Y’.
• The attribute ‘X’ can assume only those values which are assumed by the attribute ‘Y’.
• Here, the relation in which attribute ‘Y’ is present is called as the referenced relation.
• The relation in which attribute ‘X’ is present is called as the referencing relation.
• The attribute ‘Y’ might be present in the same table or in some other table.

Example-

Consider the following two schemas-

Here, t_dept can take only those values which are present in dept_no in Department table since only those departments actually exist.

NOTES-

• Foreign key references the primary key of the table.
• Foreign key can take only those values which are present in the primary key of the referenced relation.
• Foreign key may have a name other than that of a primary key.
• Foreign key can take the NULL value.
• There is no restriction on a foreign key to be unique.
• In fact, foreign key is not unique most of the time.
• Referenced relation may also be called as the master table or primary table.
• Referencing relation may also be called as the foreign table.

6. Partial Key-

• Partial key is a key using which all the records of the table can not be identified uniquely.
• However, a bunch of related tuples can be selected from the table using the partial key.

Example-

Consider the following schema-

Department ( Emp_no , Dependent_name , Relation )

Emp_no	Dependent_name	Relation
E1	Suman	Mother
E1	Ajay	Father
E2	Vijay	Father
E2	Ankush	Son

Here, using partial key Emp_no, we can not identify a tuple uniquely but we can select a bunch of tuples from the table.

7. Composite Key-

A primary key comprising of multiple attributes and not just a single attribute is called as a composite key.

8. Unique Key-

Unique key is a key with the following properties-

• It is unique for all the records of the table.
• Once assigned, its value can not be changed i.e. it is non-updatable.
• It may have a NULL value.

Example-

The best example of unique key is Adhaar Card Numbers.

• The Adhaar Card Number is unique for all the citizens (tuples) of India (table).
• If it gets lost and another duplicate copy is issued, then the duplicate copy always has the same number as before.
• Thus, it is non-updatable.
• Few citizens may not have got their Adhaar cards, so for them its value is NULL.

9. Surrogate Key-

Surrogate key is a key with the following properties-

• It is unique for all the records of the table.
• It is updatable.
• It can not be NULL i.e. it must have some value.

Example-

Mobile Number of students in a class where every student owns a mobile phone.

10. Secondary Key-

Secondary key is required for the indexing purpose for better and faster searching.

Next Article- Functional Dependency in DBMS

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Summary

Article Name

Keys in DBMS

Description

Keys in DBMS is a set of attributes that can identify each tuple uniquely of the given relation. Different Types of Keys in DBMS- Super key, Candidate key, Primary key, Alternate key, Foreign key, Partial key, Composite key, Unique key, Surrogate key, Secondary key

Author

 - Md Altaf Raja