Integrity Constraints
An integrity constraint allows us to place constraints on data being entered into/updated in a relation.
They guard against accidental damage to the database, by ensuring that authorized changes to the database do not result in a loss of data consistency.
Some common integrity constraints:
NOT NULL: the value for the attribute must not be null
UNIQUE: the value of the attribute must be unique. The specification UNIQUE() indicates that () form the candidate key
PRIMARY KEY
CHECK (P): P is a predicate; used to check if a condition is satisfied
Referential Integrity: this is needed when using foreign keys. It ensures that a value that appears in one relation for a given set of attributes also appears for a certain set of attributes in another relation
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