Tell about the keys in relational databases.

In relational databases, keys are used to identify and link data:

• Primary Key — a unique identifier for a record in a table. It cannot contain NULL and must be unique.
• Foreign Key — a field or set of fields that references the primary key of another table, establishing a relationship between the tables.
• Unique Key — ensures the uniqueness of values in a column or set of columns but may allow NULLs (depending on the DBMS).
• Composite Key — a key consisting of multiple columns that together uniquely identify a record.
• Alternate Key — any unique key that is not the primary key.

Example:

CREATE TABLE Users (
  user_id INT PRIMARY KEY,
  email VARCHAR(255) UNIQUE,
  username VARCHAR(100),
  CONSTRAINT fk_role FOREIGN KEY (role_id) REFERENCES Roles(role_id)
);