Advanced SQL Concepts - Checking of Constraints CIS 4301 Lecture Notes Lecture /6/2006

Lecture 21© CIS Spring Check Constraints Limits values that may appear in components for some attributes Expressed as either Constraint on attribute in definition of its relation’s schema Constraint on a tuple as a whole; part of relation schema, but not associated with individual attributes

Lecture 21© CIS Spring Attribute Check Constraint Simple Case: Assume in table Studio we have declaration: presC# INT REFERENCES MovieExec(cert#) NOT NULL Cannot use set-null policy to fix referential int. violation General Case: Attached to attribute declaration Keyword CHECK followed by any condition that could follow WHERE clause in SQL query Checked whenever any tuple gets a new value for this attribute (incl. on inserts of new tuples) Not checked when modification does not change the value of the attribute to which CHECK belongs

Lecture 21© CIS Spring Example CREATE TABLE MovieStar ( … gender CHAR(1) CHECK (gender IN (‘F’, ‘M’)), … ); Condition being checked can be anything that could follow WHERE in SFW query

Lecture 21© CIS Spring Example Can the following attribute-based CHECK constraint simulate a referential integrity constraint? CREATE TABLE Studio ( Name CHAR(30) PRIMARY KEY, Address VARCHAR(255), presC# INT CHECK (presC# IN (SELECT cert# FROM MovieExec)); No, updates to MovieExec are invisible to the above CHECK constraint

Lecture 21© CIS Spring Tuple-Based Check Constraint Add a tuple-based CHECK constraint to MovieStar schema that prevents the insertion of male stars whose name begin with “Ms.” Checked after insertions and updates to tuples of the relation on which it is defined CREATE TABLE MovieStar ( Name CHAR(30) PRIMARY KEY, Address VARCHAR(255), Gender CHAR(1), Birthdate DATE, CHECK (gender <> ‘M’ OR name NOT LIKE ‘Ms.%’) //forbid the insertion of tuples that satisfy multiple conditions, namely “male and name starts with ‘Ms.’ ” //equivalent to the OR of the negation of the same terms

Lecture 21© CIS Spring Assertions More powerful mechanism of constraining values in database are part of database schema First-class database citizens like views or relations Assertion is a boolean-valued SQL expression that must be true at all times Easy to state for DB implementer, simply state what must be true Harder to implement efficiently since DBMS must deduce whether or not a given database modification could affect truth of assertion

Lecture 21© CIS Spring Example Express that no one can become president of a studio unless net worth greater than $10M MovieExec(name, address, cert#, netWorth) Studio(name, address, presC#) CREATE ASSERTION RichPres CHECK (NOT EXISTS (SELECT * FROM Studio, MovieExec WHERE presC# = cert# AND netWorth < ) ) ); Can this be simulated with tuple-based CHECK constraints?

Lecture 21© CIS Spring Using a Tuple-Based CHECK CREATE TABLE Studio ( name CHAR(30) PRIMARY KEY, address VARCHAR(255), presC# INT REFERENCES MovieExec(cert#), CHECK (presc# NOT IN (SELECT cert# FROM MovieExec WHERE netWorth < ) ) );

Lecture 21© CIS Spring Example Assert that the total length of all movies by a given studio shall not exceed 10,000 minutes Movie(title,year,length,inColor,studioName,producerC#) CREATE ASSERTION SumLength CHECK (10000 >= ALL (SELECT SUM(length)FROM Movie GROUP BY StudioName)); Is the effect the same as that of the following tuple- based CHECK: CHECK (10000 >= ALL (SELECT SUM(length) FROM Movie GROUP BY studioName));

Lecture 21© CIS Spring Summary of CHECK Constraints and Assertions Type of Constraint When Declared When Activated Guaranteed to Hold Attribute-based CHECK With attribute On insertion to relation or attribute update Not if subqueries Tuple-based CHECK Element of relational schema On insertion to relation or tuple update Not if subqueries Assertion Element of Database schema On any change to any mentioned relation Yes

Lecture 21© CIS Spring Triggers Aka “event-condition-action” (ECA) rules Three important facts about triggers Only awakened when certain events, specified by db programmer, occur Executing triggers involves testing a condition first If condition satisfied, action of trigger is executed

Lecture 21© CIS Spring Example Write trigger to prevent any attempt to lower networth of movie exec MovieExec(name,address,cert#,netWorth) 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#; Event Condition Action

Lecture 21© CIS Spring Comments The action rule may be executed BEFORE or AFTER the event If before, when clause is tested before triggering event Besides update, other triggering events are insert and delete When clause is optional The action may contain any number of SQL statements, separated by BEGIN … END If triggering event is insert, may use a NEW ROW AS clause to give name to inserted row Conversely, may use OLD ROW AS in case of a deletion

Lecture 21© CIS Spring More Comments If we omit FOR EACH ROW clause, trigger becomes statement-level trigger (as opposed to row-level trigger) Statement-level trigger is executed ONCE no matter how many rows it actually effects Cannot refer to old and new tuples However, both types of triggers can access old and new set of tuples OLD TABLE AS … (i.e., deleted tuples or old versions of updated tuples) NEW TABLE AS … (i.e., inserted tuples or new versions of updated tuples)

Lecture 21© CIS Spring Example Prevent average net worth of movie executives to drop below $500K Violation on insert, update, delete => need three triggers! MovieExec(name,address,cert#,netWorth) CREATE TRIGGER AvgNetWorthTrigger AFTER UPDATE OF netWorth ON MovieExec REFERENCING OLD TABLE AS OldStuff, NEW TABLE AS NewStuff FOR EACH STATEMENT WHEN ( > (SELECT AVG(netWorth) FROM MovieExec)) BEGIN DELETE FROM MovieExec WHERE (name,address,cert#,netWorth) IN NewStuff; INSERT INTO MovieExec (SELECT * FROM OldStuff); END;