Integrity constraints for relational databases

When we want to create a relational database, we must comply with four integrity constraints in order for our DB to work correctly.

1. Entity integrity constraint. A PK (Primary Key) cannot allow null values
• Enforcement: check for null values when enter data.
2. Key integrity constraint. No two tuples can have the same attribute value.
• Enforcement: check duplicity.
3. Referential integrity constraint. We cannot have any unmatched FK (Foreign Key). That is, if B references A, A must exist.
• Enforcement:
• Restriction. The Update/delete of PK is restricted to not having a matching FK.
• Nullification. Update/delete of PK can only be done after setting any FK to null.
• Cascading. Update/delete of PK cascades to any FK.
4. Semantic integrity constraint. We must use data that makes sense. We look for semantic correctness.
• Enforcement: use restrictions when enter data such as: length, legal values (i.e. car brands), types of data (i.e. currency, date, etc.), range, etc.