Referential Integrity checks, Triggers and Assertions Examples from Chapter 7 of Database Systems: the Complete Book Garcia-Molina, Ullman, & Widom

Movie Database Movie (title, year, length, inColor, studioName, producerC#) StarsIn (movieTitle, movieYear, starName) MovieStar(name, address, gender, birthdate) MovieExec(name, address, cert#, netWorth) Studio(name, address, presC#)

Specifying Update/Delete Handling CREATE TABLE Studio ( name char(30) primary key, address varchar(255), presC# int references MovieExec(cert#) on delete set null on update cascade Deleting the corresponding MovieExec record sets presC# to Null Updating the MovieExec record modifies presC#

Not-null constraint CREATE TABLE Studio ( name char(30) primary key, address varchar(255), presC# int references MovieExec(cert#) Not Null on update cascade No longer possible to follow set-null policy on deletes to MovieExec

Attribute-value constraint CREATE TABLE Studio ( name char(30) primary key, address varchar(255), presC# int references MovieExec(cert#) Check (presC# >= ) Insertions or updates will fail if they violate the check condition

Attribute-value constraint CREATE TABLE Studio ( name char(30) primary key, address varchar(255), presC# int references MovieExec(cert#) Check (presC# in (Select cert# from MovieExec) Insertions or updates on this table will fail unless the new presC# matches an existing MovieExec However, updates or deletes on MovieExec that falsify the condition will not be stopped.

Tuple-based constraint CREATE TABLE MovieStar ( name char(30) primary key, address varchar(255), gender char(1), birthdate date, Check (gender=‘F’ or name NOT LIKE “Ms%”) Check condition is a relationship between two different attributes

Assertion CREATE Assertion RichPres CHECK (NOT EXISTS (SELECT * FROM Studio, MovieExec WHERE Studio.presC#=MovieExec.cert# AND MovieExec.netWorth< )) Check condition requires an SQL statement involving multiple tables Condition says that any president of a movie studio must be worth at least $10,000,000

Another Assertion CREATE Assertion SumLength CHECK (10000>=ALL (SELECT SUM (length) FROM Movie GROUP BY StudionName)) Assertion says that the lengths of all movies made by any studio must be no more than 10,000 minutes Note the >= ALL quantifier!

Trigger CREATE Trigger NetWorthTrigger AFTER UPDATE OF netWorth ON MovieExec REFERENCING OLD ROW AS OldTuple NEW ROW AS NewTuple FOR EACH ROW WHEN (OldTuple.netWorth>NewTuple.netWorth) UPDATE MovieExec SET netWorth = OldTuple.netWorth WHERE cert#=newTuple.cert# Prevents reducing the net worth of a movie exec Note this cannot be expressed as a constraint on tuple values!