Various DBMS Keys: Super, Primary, Candidate, Alternate, Foreign, Compound and Composite

 

Various Keys in DBMS: Super, Primary, Candidate, Alternate, Foreign, Compound and Composite

 

·      A key in DBMS is an attribute or set of attributes which helps you to identify a row (tuple) in a relation (table).

·      They allow you to find the relation between two tables.

·      Keys help you uniquely identify a row in a table by a combination of one or more columns in that table.

·      Key is also helpful for finding unique record or row from the table.

·     Database key is also helpful for finding unique record or row from the table.

 

Example:

Emp_ID	FirstName	LastName
101	Michelle	Johnson
202	Tom	Rogers
303	Alex	Wills

In the above-given example, Emp_ID is a primary key because it uniquely identifies an employee record. In this table, no other employee can have the same employee ID.

 

Why we need a Key?

Some reasons for using a key in the DBMS system are as follows:

●     Keys help you to identify any row of data in a table. In a real-world application, a table could contain thousands of records. Moreover, the records could be duplicated. Keys ensure that you can uniquely identify a table record despite these challenges.

●     Allows you to establish a relationship between and identify the relation between tables

●     Help you to enforce identity and integrity in the relationship.

Types of Keys in Database Management System:

There are mainly 7 different types of keys in DBMS and each key has its different functionality. These keys are:

●     Super Key:

It is a group of single or multiple keys which identifies rows in a table.

 

●     Primary Key:

It is a column or group of columns in a table that uniquely identify every row in that table.

 

●     Candidate Key:

A candidate key is a set of attributes that uniquely identify tuples in a table. A candidate key is a super key with no repeated attributes.

 

●     Alternate Key:

An alternate key is a column or group of columns in a table that uniquely identify every row in that table.

 

●     Foreign Key:

A foreign key is a column that creates a relationship between two tables. The purpose of foreign key is to maintain data integrity and allow navigation between two different instances of an entity.

 

●     Compound Key:

It has two or more attributes that allow you to uniquely recognize a specific record. It is possible that each column may not be unique by itself within the database.

 

●     Composite Key:

An artificial key which aims to uniquely identify each record is called a surrogate key. These kinds of keys are unique because they are created when you don't have any natural primary key.

Super key:

A super key is a group of single or multiple keys which identifies rows in a table. A super key may have additional attributes that are not needed for unique identification.

Example:

EmpSSN	EmpNum	Empname
9812345098	AB05	Shawn
9876512345	AB06	Rosy
199937890	AB07	Mark

In the above-given example, EmpSSN and EmpNum, Empname are superkeys.

Primary Key:

A primary key is a column or group of columns in a table that uniquely identify every row in that table. The primary key cannot be a duplicate; meaning the same value can't appear more than once in the table. A table cannot have more than one primary key.

Rules for defining Primary key:

●     Two rows can't have the same primary key value

●     It must for every row to have a primary key value.

●     The primary key field cannot be null.

●     The value in a primary key column can never be modified or updated if any foreign key refers to that primary key.

Example:

In the following example, StudID is a Primary Key.

StudID	Roll No	First Name	LastName	Email
1	101	Tom	Price	abc@gmail.com
2	102	Nick	Cross	pqr@gmail.com
3	103	Dana	Nathan	xyz@yahoo.com

Alternate key:

An Alternate key is a column or group of columns in a table that uniquely identify every row in that table. A table can have multiple choices for a primary key but only one can be set as the primary key. All the keys which are not primary key are called an Alternate Key.

Example:

In this table, StudID, Roll No, Email are qualified to become a primary key. But since StudID is the primary key, Roll No, Email becomes the alternative key.

StudID	Roll No	First Name	LastName	Email
1	101	Tom	Price	abc@gmail.com
2	102	Nick	Cross	pqr@gmail.com
3	103	Dana	Nathan	xyz@yahoo.com

Candidate Key:

A candidate key is a set of attributes that uniquely identify tuples in a table. Candidate key is a super key with no repeated attributes. The Primary key should be selected from the candidate keys. Every table must have at least a single candidate key. A table can have multiple candidate keys but only a single primary key.

Properties of Candidate key:

●     It must contain unique values

●     Candidate key may have multiple attributes

●     Must not contain null values

●     It should contain minimum fields to ensure uniqueness

●     Uniquely identify each record in a table

Example: In the given table Stud ID, Roll No, and email are candidate keys which help us to uniquely identify the student record in the table.

StudID	Roll No	First Name	LastName	Email
1	101	Tom	Price	abc@gmail.com
2	102	Nick	Cross	pqr@gmail.com
3	103	Dana	Nathan	xyz@yahoo.com

 

Foreign key:

A foreign key is a column that creates a relationship between two tables. The purpose of foreign keys is to maintain data integrity and allow navigation between two different instances of an entity. It acts as a cross-reference between two tables as it references the primary key of another table.

Example:

DeptCode	DeptName
001	Science
002	English
005	Computer

 

Fig: Department table

 

Teacher ID	Fname	Lname
B002	Billy	Wright
B017	Sara	Jones
B009	Mike	Williams

 

Fig: Teacher table

 

In ex:, we have two table, teacher and department in a school. However, there is no way to see which teacher work in which department.

In this table, adding the foreign key in Deptcode to the Teacher name, we can create a relationship between the two tables.

Teacher ID	DeptCode	Fname	Lname
B002	002	Billy	Wright
B017	002	Sara	Jones
B009	001	Mike	Williams

 

Fig: Foreign key relationship

This concept is also known as Referential Integrity.

Compound key:

The compound key has two or more attributes that allow you to uniquely recognize a specific record. It is possible that each column may not be unique by itself within the database. However, when combined with the other column or columns the combination of composite keys become unique. The purpose of the compound key in database is to uniquely identify each record in the table.

Example:

OrderNo	ProductID	Product Name	Quantity
B005	JAP102459	Mouse	5
B005	DKT321573	USB	10
B005	OMG446789	LCD Monitor	20
B004	DKT321573	USB	15

 

In this ex:, OrderNo and ProductID can't be a primary key as it does not uniquely identify a record. However, a compound key of Order ID and Product ID could be used as it uniquely identified each record.

Composite key:

The composite key is a combination of two or more columns that uniquely identify rows in a table. The combination of columns guarantees uniqueness, though individually uniqueness is not guaranteed. Hence, they are combined to uniquely identify records in a table.

The difference between compound and the composite key is that any part of the compound key can be a foreign key, but the composite key may or maybe not a part of the foreign key.