Explain what is meant by constraints in the context of databases.

In the context of databases, constraints are rules imposed on data in tables to ensure their integrity and correctness. Main types of constraints:

• PRIMARY KEY — a unique identifier for a record.
• FOREIGN KEY — a reference to a key from another table, ensuring a relationship between tables.
• UNIQUE — guarantees the uniqueness of a value in a column.
• NOT NULL — prohibits storing empty values.
• CHECK — verifies that a value satisfies a certain condition.

For example, a FOREIGN KEY constraint will prevent adding a record with a non-existent identifier in the related table, thus preventing relationship violations.