There are basically 4 types of main constraints in SQL:
Domain Constraint: if one of the attribute values provided for a new
tuple is not of the specified attribute domain
Key Constraint: if the value of a key attribute in a new tuple
already exists in another tuple in the relation
Referential Integrity: if a foreign key value in a new tuple
references a primary key value that does not exist in the referenced
relation
Entity Integrity: if the primary key value is null in a new tuple
