Types Of Keys In DBMS

keys are attributes or combinations of attributes that uniquely identify each record (or row) within a table. Keys play a crucial role in ensuring data integrity, enforcing constraints, and facilitating efficient data retrieval and manipulation. Keys define the structure and relationships of data within a database and are used to establish connections between different tables. There are several types of keys in DBMS, including primary keys, candidate keys, alternate keys, foreign keys, and composite keys.

In the below PDF we discuss about Types of Keys in DBMS in detail in simple language, Hope this will help in better understanding.

Types of DBMS Keys:

1. Primary Key (PK):

  • The primary key is a unique identifier for each record in a table.
  • It ensures that each row in a table is distinct and can be uniquely identified.
  • Primary keys enforce entity integrity, preventing duplicate or null values in the key field.
  • Typically, primary keys are implemented using single or composite columns.
    Example: A user ID in a user table can serve as a primary key.

2. Foreign Key (FK):

  • A foreign key establishes a link between two tables, referencing the primary key of another table.
  • It ensures referential integrity by enforcing relationships between related tables.
  • Foreign keys help maintain data consistency and integrity across the database.
    Example: A foreign key in an order table referencing the primary key of a customer table establishes a relationship between orders and customers.

3. Unique Key:

  • A unique key ensures that all values in a column or a set of columns are distinct and not null.
  • Unlike primary keys, unique keys allow null values.
  • They prevent duplicate entries but do not necessarily serve as identifiers for the entire table.
  • A table can have multiple unique keys.
    Example: An email address column in a user table can be enforced as a unique key to prevent duplicate emails.

4. Composite Key:

  • A composite key is formed by combining two or more columns to create a unique identifier for a record.
  • It is used when a single column cannot uniquely identify a record.
    Composite keys are useful in many-to-many relationships or tables without a suitable single-column primary key.
    Example: A combination of ‘product ID’ and ‘store ID’ in a sales table can form a composite key to uniquely identify each sales transaction.

5. Candidate Key:

  • A candidate key is a set of one or more columns that can uniquely identify a record in a table.
  • From the set of candidate keys, one is chosen as the primary key.
  • All candidate keys have the unique property but may have different meanings.
    Example: In a student table, both ‘student ID’ and ‘social security number’ can serve as candidate keys.

6. Super Key:

  • A super key is a set of columns that uniquely identifies each record in a table.
  • It can include more columns than necessary to form a minimal unique identifier.
  • Super keys are not necessarily minimal, meaning they can contain extraneous columns.
    Example: In a customer table, a super key can include ‘customer ID’ along with ‘first name’ and ‘last name’.


In Conclusion, Understanding the various types of keys in DBMS is essential for designing efficient and robust database schemas. By appropriately utilizing these keys, database designers can ensure data integrity, establish relationships between entities, and optimize data retrieval and manipulation operations. Each type of key serves a specific purpose, contributing to the overall effectiveness and reliability of the database system.

Related Question

A primary key is a unique identifier for each record in a database table. It ensures that each row within the table is uniquely identifiable and cannot contain duplicate values.

A primary key uniquely identifies each record in a table, whereas a foreign key establishes a relationship between two tables by referencing the primary key of another table.

A composite key is a combination of two or more columns in a table that uniquely identifies each record. It’s used when a single column cannot guarantee uniqueness.

A foreign key constraint is a rule that enforces referential integrity between two related tables in a database. It ensures that values in a column (or a set of columns) in one table match values in another table’s primary key or unique key.

Indexing is a technique used to improve the speed of data retrieval operations on a database table. Keys, such as primary keys and foreign keys, are often indexed to facilitate faster searching and retrieval of data.


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