Keys in DBMS

Keys

A key is an attribute or set of attributes that help to uniquely identify a record or a row of data in a table(relation).

Why do we need Keys?

1. Keys are used to establish and identify relationships between tables/relations.
2. They uniquely identify any record/row inside a table/relation.
3. They also ensure that data integrity is maintained.

Types keys in DBMS/RDBMS 

The different types of keys in DBMS/RDBMS are:

• Super Key
• Candidate Key
• Primary Key
• Alternate Key
• Foreign Key

For understanding the different types of keys better, let us consider the below table. In this ‘Student’ table, ID, Registration_No. and Name of students are given.

Super Key

If a set of one or more attributes can uniquely identify the entities of an entity set, then it is called a super key.

Example: In the above table the super keys are

{ ID }

{Registration_No.}

{ ID, Registration_No.}

{ID, Name} 

{Registration_No. ,Name}

{ID, Registration_No., Name}

NOTE: {Name} is not a super key as there is more than one record/row with the same name.

Candidate Key

Candidate keys are selected from a set of super keys. If a super key has no unnecessary attributes, then such a minimal super key is called a candidate key. It is also termed as ‘Minimal Super Key’. 

Example: In the student entity set {ID}, {Registration_No.},  {ID, Registration_No.} etc.  are candidate keys. But {ID, Name} is not a candidate key, because {Name} is a redundant attribute.

Primary Key

Primary keys are selected from a set of candidate keys. It is a candidate key selected by the database designer to uniquely identify the records of a table. 

• The primary key field of a table must be unique and cannot be null. 
• A table does not contain more than one primary key.

Alternate Key

If a candidate key is not selected as a primary key then it is called an alternate key. 

Example: If {ID} is selected as primary key, then {Registration_No.} can be selected as an alternate key

Foreign Key

If two relations or tables have a common attribute, which is the primary key in one table and a non-key attribute in another table, then such a non-key attribute is called a foreign key. 

• A table can contain zero or more foreign keys.
• It helps to combine the tuples of two tables during the join operation.

In the above table, {ID} is the primary key in ‘Student’ Table, but foreign key in the ‘Marks’ Table.

Also Read - Cardinality In DBMS

Advantages of Using Keys in DBMS

• Ensures Uniqueness of Records
  Keys like primary keys ensure that each row in a table is unique. For example, a student_id in a student table uniquely identifies every student.
   
• Maintains Data Integrity
  Keys prevent duplicate or null values in key columns. This ensures reliable and accurate data storage across the database.
   
• Supports Data Retrieval Efficiency
  Using keys helps the DBMS locate data quickly during queries. For instance, indexing on a key speeds up search operations like SELECT queries.
   
• Helps in Establishing Relationships Between Tables
  Foreign keys create links between related tables, such as linking orders to customers. This supports relational integrity and organized data structure.
   
• Simplifies Data Management and Indexing
  Keys aid in automatic indexing, making data access faster and easier. This improves the performance of operations like sorting, searching, and joining.

Disadvantages of Using Keys in DBMS

• Increased Complexity in Table Design
  Defining multiple keys requires careful planning, especially when handling composite or foreign keys, which can complicate schema design.
   
• Potential Performance Overhead with Multiple Key Constraints
  Too many keys or constraints can slow down insert or update operations due to frequent integrity checks.
   
• Foreign Key Dependencies Can Complicate Deletion/Update
  When a table has foreign key references, deleting or updating records in the parent table may result in errors or require cascading changes.
   
• Rigid Structure in Dynamic Environments
  In highly dynamic databases, keys can restrict flexibility, making it difficult to restructure tables without breaking relationships.
   
• Duplicate Data Issues if Keys Are Poorly Defined
  Without well-chosen keys, duplicates may still occur, leading to data inconsistencies or query errors.

Frequently Asked Questions

Q1 What are keys in DBMS?

Ans: A key is an attribute or set of attributes that help to uniquely identify a record(row) of data in a table(relation).

The different types of keys are: 

• Super Key
• Candidate Key
• Primary Key
• Alternate Key
• Foreign Key

Q2 Explain “Every candidate key is a super key, but the reverse is not always true”.

Ans: If a set of one or more attributes can uniquely identify the entities of an entity set, then it is called a super key. 

From the list of super keys, the keys which have no unnecessary attributes are the candidate keys. So every candidate key is essentially a super key, but every super key is not always a candidate key.

Q3 Explain ACID properties in DBMS.

Ans: ACID properties are four properties that are followed before and after a transaction is made in the database. ACID properties maintain the consistency of data in the database.

The ACID properties are,

• Atomicity
• Consistency
• Isolation
• Durability

Q4 What is a Join operation?

Ans: Join Operation is a binary operation, which can be applied on two relations/ tables having at least one common attribute. It produces a new relation with a combination of tuples. 

There are three types of join operations:

1. Theta Join
2. Equi Join
3. Natural Join

Conclusion

In this blog, we discussed Keys in DBMS. We saw why it is important in DBMS and also the different types of keys present. We hope you learned something new in this blog. 

Recommended Readings:

• Attributes in DBMS
• Types of DBMS
• Super Keys in DBMS