What is the role of keys in a database structure and why are they used?

Keys in a database structure are special fields (or sets of fields) used for data identification and organization.

Main types of keys:

• Primary Key: uniquely identifies each record in a table. Cannot contain NULL and must be unique.
• Foreign Key: a field that references the primary key of another table, ensuring a relationship between tables.
• Unique Keys: guarantee the uniqueness of values in a column but can allow NULL.

Why keys are needed:

• Ensure data integrity (e.g., prevent duplicate records).
• Enable efficient searching and linking of data between tables.
• Help optimize queries and indexing.

Example: in the users table, the id field can be a primary key, and in the orders table, the user_id field can be a foreign key referencing users.id.