Constraints specify rules for data in a SQL database. They ensure that data in a database remains valid. Some common constraints for RDBMSs include:
- Not Null: Prevents storing a NULL value in a column.
- Unique: Prevents a duplicate value of, for example, a primary key.
- Primary Key: A combination of a NOT NULL and UNIQUE. Ensures that a key field contains a unique value.
- Foreign Key: Ensures there is an actual value in the referenced table; one that cannot be deleted.
- Check Constraint: Allows only specific values in a field.
- Default: Sets a default value for a field.