Integrity constraints are rules enforced by the database to ensure data validity and consistency. They prevent invalid or inconsistent data and capture business rules.
- Primary key
Uniquely identifies a row and cannot be NULL. - Foreign key
Enforces referential integrity between tables. - Unique
Ensures column values are unique. - Not null
Disallows NULL values in a column. - Check
Enforces custom conditions or ranges on values.
Common Integrity Constraints
PRIMARY KEY
Declare the primary key of the relation. If multiple columns are used, they are declared as a composite primary key.
NOT NULL
Declare an attributes to be not null. Applied to primary keys automatically.
UNIQUE
Declare a column or set of columns to be unique. Can be null.
CHECK
Enforce custom conditions or ranges on values. All kinds of WHERE predicates are allowed with CHECK. Can refer multiple columns of a single table. Cannot reference other rows or other tables.
DEFAULT
Specify a default value for a column.
Referential Integrity
Enforce referential integrity between tables. Foreign keys must reference existing primary keys.