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Constraints in a database or system are used to ensure data integrity and proper functioning of the application. An example of a constraint in a database can be:

• Uniqueness (UNIQUE) — ensures that values in a column or set of columns do not repeat.
• Primary key (PRIMARY KEY) — uniquely identifies each record in the table.
• Foreign key (FOREIGN KEY) — ensures referential integrity between tables.
• NOT NULL — prohibits storing null values in a column.

For example, in a users table, a uniqueness constraint can be imposed on the email field to prevent registration of multiple accounts with the same address.