Integrity Constraints Notes

Null Values and Three Valued Logic

Null means the absence of a value, not zero and not empty. Comparisons with null produce unknown, which turns two valued logic into three and changes...

Foreign Keys and Referential Actions

What happens to child rows when a referenced parent row is deleted or updated. Cascade, restrict, set null, set default and no action, with the rule f...

Entity, Referential and Domain Integrity

The three integrity rules of the relational model: a primary key may not be null, a foreign key must match an existing primary key or be null, and eve...