SQL- Introduction to advanced sql concepts

Introduction to AdvancedIntroduction to Advanced
SQL Concepts - CheckingSQL Concepts - Checking
of Constraintsof Constraints

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

Attribute CheckAttribute Check
ConstraintConstraint
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
o Checked whenever any tuple gets a new value for this attribute (incl.
on inserts of new tuples)
o Not checked when modification does not change the value of the
attribute to which CHECK belongs

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

ExampleExample
• 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

Tuple-Based CheckTuple-Based Check
ConstraintConstraint• Add a tuple-based CHECK constraint to MovieStar
schema that prevents the insertion of male stars
whose name begin with “Ms.”
o 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

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

ExampleExample
• 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 < 10000000)
)
);
• Can this be simulated with tuple-based CHECK
constraints?

Using a Tuple-BasedUsing a Tuple-Based
CHECKCHECKCREATE 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 < 10000000)
)
);

ExampleExample
• 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));

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

ExampleExample• 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

CommentsComments
• The action rule may be executed BEFORE or AFTER the event
o 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
o Conversely, may use OLD ROW AS in case of a deletion

More CommentsMore 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
o Cannot refer to old and new tuples
• However, both types of triggers can access old and new set
of tuples
o OLD TABLE AS … (i.e., deleted tuples or old versions of
updated tuples)
o NEW TABLE AS … (i.e., inserted tuples or new versions of
updated tuples)

ExampleExample
• 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 (500000 > (SELECT AVG(netWorth) FROM MovieExec))
BEGIN
DELETE FROM MovieExec
WHERE (name,address,cert#,netWorth) IN NewStuff;
INSERT INTO MovieExec
(SELECT * FROM OldStuff);
END;

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SQL- Introduction to advanced sql concepts

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SQL- Introduction to advanced sql concepts