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’.