What is a Key in the Database?

In a database, a key is a field or combination of fields that uniquely identifies a record in a table. Keys play a crucial role in establishing relationships between tables and ensuring the integrity and efficiency of the database. There are different types of keys, each serving a specific purpose:

Primary Key:

• A primary key is a unique identifier for a record within a table.
• It must have a unique value for each record and cannot contain null values.
• Every table typically has one primary key.
• Examples: Social Security Number, Student ID.

Foreign Key:

• A foreign key is a field in a table that is a primary key in another table.
• It establishes a link or relationship between the tables.
• Used to maintain referential integrity between related tables.
• Examples: Employee ID in an Orders table, referencing the Employee table's Employee ID.

Composite Key:

• A composite key is a combination of two or more fields that together uniquely identify a record.
• Useful when a single field cannot uniquely identify records.
• Examples: Combination of CustomerID and OrderDate.

Unique Key:

• A unique key is similar to a primary key but allows null values.
• It enforces the uniqueness of values but does not automatically imply the "primary" role.
• Useful when you want a unique constraint but not necessarily a primary key.
• Examples: Email address in a Users table.

Importance of Keys:

• Data Integrity: Keys ensure that each record in a table can be uniquely identified, preventing data duplication and maintaining accuracy.
• Relationships: Keys establish relationships between tables, enabling the creation of links between related data in different tables.
• Efficiency: Indexing keys can significantly improve the speed of data retrieval operations.

In summary, keys in a database are essential for uniquely identifying records, establishing relationships between tables, and maintaining the integrity and efficiency of the database structure.

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