# Integrity Constraints An integrity constraint allows us to place constraints on data being entered into/updated in a relation. They guard against accidental damage to the database, by ensuring that authorized changes to the database do not result in a loss of data consistency. Some common integrity constraints: * **NOT NULL**: the value for the attribute must not be null * **UNIQUE**: the value of the attribute must be unique. The specification UNIQUE($$A\_1, A\_2, ..., A\_n$$) indicates that ($$A\_1, A\_2, ..., A\_n$$) form the candidate key * **PRIMARY KEY** * **CHECK (P)**: P is a predicate; used to check if a condition is satisfied * **Referential Integrity**: this is needed when using foreign keys. It ensures that a value that appears in one relation for a given set of attributes also appears for a certain set of attributes in another relation --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://vikram-bajaj.gitbook.io/cs-gy-6083-principles-of-database-systems/main-1/sql/intermediate-sql/integrity-constraints.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.