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
Section titled “Common Integrity Constraints”PRIMARY KEY
Section titled “PRIMARY KEY”Declare the primary key of the relation. If multiple columns are used, they are declared as a composite primary key.
NOT NULL
Section titled “NOT NULL”Declare an attributes to be not null. Applied to primary keys automatically.
UNIQUE
Section titled “UNIQUE”Declare a column or set of columns to be unique. Can be null.
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
Section titled “DEFAULT”Specify a default value for a column.
Referential Integrity
Section titled “Referential Integrity”Enforce referential integrity between tables. Foreign keys must reference existing primary keys.