m.p. johnson, dbms, stern/nyu, sp20041 c20.0046: database management systems lecture #13 matthew p....

M.P. Johnson, DBMS, Stern/NYU, Sp2004

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C20.0046: Database Management SystemsLecture #13

Matthew P. Johnson

Stern School of Business, NYU

Spring, 2004


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Agenda Last time: G&A, modifications, defining

schemata, indices This time:, Constraints, Triggers

1. Views

2. Constraints

3. Triggers Homework 2 is due now Future:

Lots of programming for SQL


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Quick review Examples from sqlzoo.net


4

New topic: Views (6.7) Stored relations physically exist and persist Views are relations that don’t

in text, “table” = stored relation = “base table” Basically names/references given to queries

maybe a relevant subset of a table Employee(ssn, name, department, project, salary)

Payroll has access to Employee, others only to Developers

CREATE VIEW Developers AS SELECT name, project FROM Employee WHERE department = ‘Development’



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A Different View Person(name, city) Purchase(buyer, seller, product, store) Product(name, maker, category)

We have a new virtual table:NYC-view(buyer, seller, product, store)

CREATE VIEW NYC-view AS SELECT buyer, seller, product, store FROM Person, Purchase WHERE Person.city = ‘New York’ AND Person.name = Purchase.buyer



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A Different View

Now we can query the view:

SELECT name, storeFROM NYC-view, ProductWHERE NYC-view.product = Product.name AND Product.category = ‘shoes’

SELECT name, storeFROM NYC-view, ProductWHERE NYC-view.product = Product.name AND Product.category = ‘shoes’




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What happens when we query a view ? SELECT name, NYC-view.store

FROM NYC-view, Product WHERE NYC-view.product = Product.name AND Product.category = ‘shoes’

SELECT name, NYC-view.store FROM NYC-view, Product WHERE NYC-view.product = Product.name AND Product.category = ‘shoes’

SELECT name, Purchase.storeFROM Person, Purchase, ProductWHERE Person.city = ‘NYC’ AND Person.name = Purchase.buyer AND Purchase.product = Product.name AND Product.category = “shoes”

SELECT name, Purchase.storeFROM Person, Purchase, ProductWHERE Person.city = ‘NYC’ AND Person.name = Purchase.buyer AND Purchase.product = Product.name AND Product.category = “shoes”


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Can rename view fields

Views info: USER_VIEWS

CREATE VIEW NYC-view(nycbuyer, nycseller, prod, store) AS

SELECT buyer, seller, product, store FROM Person, Purchase WHERE Person.city = ‘NYC’ AND Person.name = Purchase.buyer

CREATE VIEW NYC-view(nycbuyer, nycseller, prod, store) AS

SELECT buyer, seller, product, store FROM Person, Purchase WHERE Person.city = ‘NYC’ AND Person.name = Purchase.buyer


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More complex views NYC-View is just a subset of a table Views can be arbitrarily complex Imagine the database of student and course

records Students get one view:

For each semester, My-courses Professors get another view

For each course, My-students Bursar gets another view, etc.


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Types of Views Views discussed here:

Used in databases Computed only on-demand – slow at runtime Always up to date

Sometimes talk about “materialized” views Used in data warehouses Pre-computed offline – fast at runtime May have stale data More later…


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Updating Views How can I insert a tuple into a table that doesn’t exist? Employee(ssn, name, department, project, salary)



INSERT INTO Developers VALUES(‘Conrad’, ‘Optimizer’)

INSERT INTO Developers VALUES(‘Conrad’, ‘Optimizer’)

INSERT INTO Employee(ssn, name, department, project, salary)VALUES(NULL, ‘Conrad’, NULL, ‘Optimizer’, NULL)

INSERT INTO Employee(ssn, name, department, project, salary)VALUES(NULL, ‘Conrad’, NULL, ‘Optimizer’, NULL)

If we make thefollowing insertion:

It becomes:


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Non-Updatable Views Person(name, city) Purchase(buyer, seller, product, store)

How can we add the following tuple to the view?(‘NYC’, ‘Nine West’)

We don’t know the name of the purchaser cannot set to NULL (why?) To delete:

CREATE VIEW City-Store AS SELECT Person.city, Purchase.store FROM Person, Purchase WHERE Person.name = Purchase.buyer

CREATE VIEW City-Store AS SELECT Person.city, Purchase.store FROM Person, Purchase WHERE Person.name = Purchase.buyer

DROP VIEW City-StoreDROP VIEW City-Store


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New topic: Constraints & Triggers (7) Certain properties we’d like our database to hold

Enforce application semantics e.g., sid is a key

prevent inconsistencies e.g., sname has to be a string, age must be < 200

Modification of tables may break these properties Constrains enforced by either

Preventing actions causing illegal states Performing compensating actions

Triggers are one option


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Constraints in SQL Domain constraints Keys Foreign keys/referential integrity Attribute-level constraints Tuple-level constraints Global constraints: assertions

The more complex, the harder/more expensive to check and enforce

simplest

Mostcomplex


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Denoting primary keys In the CREATE TABLE statement, use:

PRIMARY KEY or UNIQUE Example:

If NAME and ADDRESS are together the primary key, it must be written as:

CREATE TABLE MovieStar (name VARCHAR(30) PRIMARY KEY,address VARCHAR(255),gender CHAR(1)

)

CREATE TABLE MovieStar (name VARCHAR(30) PRIMARY KEY,address VARCHAR(255),gender CHAR(1)

)

CREATE TABLE MovieStar (name VARCHAR(30),address VARCHAR(255),gender CHAR(1),PRIMARY KEY (name, address) )

CREATE TABLE MovieStar (name VARCHAR(30),address VARCHAR(255),gender CHAR(1),PRIMARY KEY (name, address) )


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Effects of primary keys No two tuples may agree on all key atts

Have key no duplicates at all No nulls allowed in any of the key atts Only one primary key is allowed per table

NB: SQL key is slightly different from Relational Model key In R.M., key fields values all other field values In SQL, key field values really are unique

NB: no concept of superkey in SQL Of course, chosen keys should be minimal (why?)


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CREATE TABLE Product ( productID CHAR(10),

name CHAR(30),category VARCHAR(20),price INT,UPC CHAR(20) UNIQUE,

PRIMARY KEY (productID), UNIQUE (name, category))

CREATE TABLE Product ( productID CHAR(10),

name CHAR(30),category VARCHAR(20),price INT,UPC CHAR(20) UNIQUE,

PRIMARY KEY (productID), UNIQUE (name, category))

There is at most one PRIMARY KEY;there can be many UNIQUE

Multiple keys


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PRIMARY KEY v. UNIQUE Can use the UNIQUE keyword (instead of PRIMARY KEY) in

(almost) same way Unique keys are…not unique NULLs allowed, and multiple NULLs allowed (why?)

Example:

If NAME and ADDRESS are together unique, we can write:

CREATE TABLE MovieStar (name VARCHAR(30) UNIQUE,address VARCHAR(255),gender CHAR(1) )

CREATE TABLE MovieStar (name VARCHAR(30) UNIQUE,address VARCHAR(255),gender CHAR(1) )

CREATE TABLE MovieStar (name VARCHAR(30),address VARCHAR(255),gender CHAR(1),UNIQUE (name, address) )

CREATE TABLE MovieStar (name VARCHAR(30),address VARCHAR(255),gender CHAR(1),UNIQUE (name, address) )


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Indices & keys Close affinity between indices and keys Want to keep values unique (key) need

easy way to check for existing values (index) In many implementations (like Oracle):

creating keys (primary or unique) automatically creates indices

Explicit unique index creation:

In effect, automatically creates key Index info: USER_INDEXES

CREATE UNIQUE INDEX MyIndex on MyTable(a,b,c)CREATE UNIQUE INDEX MyIndex on MyTable(a,b,c)


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Foreign Key Constraint ActedIn(ActorName, MovieName) Movies(MovieName, year) If MovieName in ActedIn is a foreign key for

Movies, then: 1. The referenced atts must be a key in the ref-ed table

2. All non-null foreign key values must appear in the referenced table

ActorName MovieName MovieYear

Title Year

(1)

(2)ActedIn

Movies


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Foreign key example Recall, ActedIn has FK MovieName...

Movies(MovieName, year) (Fatal Attraction, 1987) ActedIn(ActorName, MovieName) (Michael Douglas, Fatal Attraction)


M.Douglas Fatal Attract 1987

Title Year

Fatal Attraction 1987

(1)

(2)ActedIn

Movies


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Declaring Foreign Key Constraints REFERENCES keyword:

Or summarize at end:

CREATE TABLE ActedIn (ActorName VARCHAR(30),MovieName VARCHAR(30)

REFERENCES Movies(MovieName) )

CREATE TABLE ActedIn (ActorName VARCHAR(30),MovieName VARCHAR(30)

REFERENCES Movies(MovieName) )

CREATE TABLE ActedIn (ActorName VARCHAR(30),MovieName VARCHAR(30),CONSTRAINT const_nameFOREIGN KEY (MovieName) REFERENCES Movies(MovieName))

CREATE TABLE ActedIn (ActorName VARCHAR(30),MovieName VARCHAR(30),CONSTRAINT const_nameFOREIGN KEY (MovieName) REFERENCES Movies(MovieName))


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Constraint example Q: Is this the same as REFERENCES?

CREATE TABLE StarrtedIn (

ActorName VARCHAR(30),

MovieName VARCHAR(30)

CHECK (MovieName IN

(SELECT MovieName FROM Movies) ) )

CREATE TABLE StarrtedIn (



CHECK (MovieName IN

(SELECT MovieName FROM Movies) ) )


M.Douglas Fatal Attract 1987

Title Year

Fatal Attraction 1987

(1)

(2)ActedIn

Movies


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Dangling tuples result from Can result from:

In StarredIn: insert/update In Movies: delete/update

Responses:1. Reject

2. Cascade

3. Set-null


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What happens during updates? SQL has three policies for maintaining

referential integrity: Reject violating modifications (default) Cascade: after a delete/update do a

delete/update Set-null set foreign-key field to NULL


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Cascading and set-null Cascading: Applies to last two causes:

Deleting exec Bad exec update I.e., loss/change of exec referred to

Strategy: del/change exec referred to del/change the reference to exec

Loss/change of exec referred to set reference to null


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Cascading and set-null Cascade/set-null choices independent by att:

CREATE TABLE Studio (name CHAR(30) PRIMARY KEY,address VARCHAR(255),presC# int REFERENCES MovieExec(cert#)

ON DELETE SET NULLON UPDATE CASCADE

)

CREATE TABLE Studio (name CHAR(30) PRIMARY KEY,address VARCHAR(255),presC# int REFERENCES MovieExec(cert#)

ON DELETE SET NULLON UPDATE CASCADE

)


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Circular dependencies Sometimes one table refers to another and

the other refers to the first Neither can be inserted without the other

Soln involves Transactions (xacts) Deferring constraint check until end of xact

Constraints can be NOT DEFERABLE (default) or DEFERABLE DEFERABLE INITIALLY DEFERRED DEFERABLE INITIALLY IMMEDIATE


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Deferring constraints

CREATE TABLE Studio (name CHAR(30) PRIMARY KEY,address VARCHAR(255),presC# INT UNIQUE

REFERENCES MovieExec(cert#)DEFERRABLE INITIALLY DEFERRED

)

CREATE TABLE Studio (name CHAR(30) PRIMARY KEY,address VARCHAR(255),presC# INT UNIQUE

REFERENCES MovieExec(cert#)DEFERRABLE INITIALLY DEFERRED

)


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Constraints Constraints can be applied to

1. On attributes

2. On tuples Set update and delete actions inde’ly:

ON UPDATE not supported by Oracle

CREATE TABLE StarredIn (



REFERENCES Movies(MovieName)

ON DELETE SET CASCADE )

CREATE TABLE StarredIn (



REFERENCES Movies(MovieName)

ON DELETE SET CASCADE )


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Constraining Attribute Values Constrain invalid values

Any test legal in a WHERE clause

Subqueries (but not in Oracle!), etc.

The constraint is checked whenever the local attributes it references are changed or added

name CHAR(30) NOT NULLname CHAR(30) NOT NULL

gender CHAR(1) CHECK (gender IN (‘F’, ‘M’))gender CHAR(1) CHECK (gender IN (‘F’, ‘M’))


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CREATE TABLE Purchase (prodName CHAR(30)

CHECK (prodName IN SELECT Product.name FROM Product), date DATETIME NOT NULL)

CREATE TABLE Purchase (prodName CHAR(30)

CHECK (prodName IN SELECT Product.name FROM Product), date DATETIME NOT NULL)

Is this the same as a foreign key?


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Naming domains to constrain values Can define new domains to use as the

attribute type:

Then update our attribute definition... gender GenderDomain

Note use of VALUE to refer to the att value Named domains are not supported by Oracle

CREATE DOMAIN GenderDomain CHAR(1)CHECK (VALUE IN (‘F’, ‘M’))

CREATE DOMAIN GenderDomain CHAR(1)CHECK (VALUE IN (‘F’, ‘M’))


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Among several attributes in one table Specify at the end of

CREATE TABLE

CHECK (gender = ‘F’ OR name NOT LIKE ‘Ms.%’)

Checked whenever a tuple of the relation is added or updated.

CREATE TABLE Sailors( sid INTEGER,sname CHAR(10),rating INTEGER,age REAL,PRIMARY KEY (sid),CHECK ( rating >= 1 AND rating <= 10)

)

CREATE TABLE Sailors( sid INTEGER,sname CHAR(10),rating INTEGER,age REAL,PRIMARY KEY (sid),CHECK ( rating >= 1 AND rating <= 10)

)

More complex constraints


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Adding/deleting named constraints Add:

Delete:

Constraints info: USER_CONSTRAINTS

ALTER TABLE MovieADD CONSTRAINT Const1 PRIMARY KEY(name)

ALTER TABLE MovieADD CONSTRAINT Const1 PRIMARY KEY(name)

ALTER TABLE MovieDROP CONSTRAINT Const1

ALTER TABLE MovieDROP CONSTRAINT Const1


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Assertions (7.4) Constraint on entire database (not table!)

Checked upon any change to DB Not supported by Oracle

CREATE ASSERTION <name> CHECK (<condition>) Analogous to our assertions in R.A.: Studio(Name, Address, PresC#); MovieExec(Name, Address, Cert#, NetWorth);

CREATE ASSERTION RichPres CHECK(NOT EXISTS

(SELECT * FROM Studio, MovieExec WHERE presC# = cert# AND netWorth < 10,000,000))

CREATE ASSERTION RichPres CHECK(NOT EXISTS

(SELECT * FROM Studio, MovieExec WHERE presC# = cert# AND netWorth < 10,000,000))


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Another assertion example Movie(Title, Year, Length, inColor, StudioName,

ProducerC#)

Q: Can we write this as a tuple constraint? Q: Could we if >= < ?

CREATE ASSERTION SumLength CHECK (10000 >= ALL (SELECT SUM(length) FROM Movie GROUP BY studioName))

CREATE ASSERTION SumLength CHECK (10000 >= ALL (SELECT SUM(length) FROM Movie GROUP BY studioName))


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Another example Q: What does this check?

CREATE ASSERTION myAssert CHECK NOT EXISTS(

SELECT Product.nameFROM Product, PurchaseWHERE Product.name = Purchase.prodNameGROUP BY Product.nameHAVING count(*) > 200)

CREATE ASSERTION myAssert CHECK NOT EXISTS(

SELECT Product.nameFROM Product, PurchaseWHERE Product.name = Purchase.prodNameGROUP BY Product.nameHAVING count(*) > 200)


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Different Constraint Types

Q: Why not always use assertions?

Type Where Declared

When activated

Guaranteed to hold?

Attribute with attribute on insertion or update

not if subquery

Tuple relation schema

insertion or update to relation

not if subquery

Assertion database schema

on change to any relation mentioned

Yes


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Triggers Constraints state what must remain true

DBMS decides when to check Triggers are instructions to perform at explicitly

specified times Three aspects:

An event (e.g., update to an attribute) A condition (e.g., a query to check) An action (the trigger’s effect) (deletion, update, insertion)

When the event occurs, DBMS checks the constraint, and if it is satisfied, performs the action


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Triggers – important points Can replace old row (result of event) with

new row Action may be performed before or after

event Can refer to old row and new row WHEN clauses tests whether to continue Action may be performed either

For each row involved in event Once per event

Oracle does triggers as PL/SQL programs


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Elements of Triggers Timing of action execution: before, after or instead

of triggering event The action can refer to both the old and new state of

the database Update events may specify a particular column or

set of columns A condition is specified with an optional WHEN

clause The action can be performed either for

once for every tuple or once for all the tuples that are changed by the database

operation


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Simple trigger example R(id, data, last-modified)

data is a large string Last-modified is a newly added date field

Goal: whenever data is modified, update last-modified date

Could modify all scripts/programs that touch this table Bad idea

Better: user a trigger

CREATE TRIGGER UpdateDateTriggerBEFORE UPDATE OF data ON R

REFERENCING

NEW ROW AS NewTupleFOR EACH STATEMENTBEGIN

NewTuple.last-modified = sysdate;END;

CREATE TRIGGER UpdateDateTriggerBEFORE UPDATE OF data ON R

REFERENCING

NEW ROW AS NewTupleFOR EACH STATEMENTBEGIN

NewTuple.last-modified = sysdate;END;


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Triggers: Row-level example MovieExec(name, address, cert#, netWorth) “If someone decreases a movie executive’s net worth, I

want the database to reset itself to the previous net worth.”

CREATE TRIGGER NetWorthTrigger

AFTER UPDATE OF netWorth ON MovieExec

REFERENCING

NEW ROW AS NewTuple

OLD ROW AS OldTuple

FOR EACH ROW

WHEN (OldTuple.netWorth>NewTuple.netWorth)

UPDATE MovieExec

SET netWorth = oldTuple.netWorth

WHERE cert# = newTuple.cert#)

CREATE TRIGGER NetWorthTrigger

AFTER UPDATE OF netWorth ON MovieExec

REFERENCING

NEW ROW AS NewTuple

OLD ROW AS OldTuple

FOR EACH ROW

WHEN (OldTuple.netWorth>NewTuple.netWorth)

UPDATE MovieExec

SET netWorth = oldTuple.netWorth

WHERE cert# = newTuple.cert#)


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Triggers: Table-level example MovieExec(name, address, cert#, netWorth) “If someone updates the net worth of one movie exec so that the

average net worth of all movie execs becomes less than $50,000, I want the database to reset itself.”

CREATE TRIGGER AvgNetWorthTriggerAFTER UPDATE OF netWorth ON MovieExecREFERENCING

OLD TABLE AS OldStuff,NEW TABLE AS NewStuff

FOR EACH STATEMENTWHEN (50000 > (SELECT AVG(netWorth) FROM MovieExec))BEGIN

DELETE FROM MovieExecWHERE (Name, address, cert#, netWorth) IN

NewStuff;INSERT INTO MovieExec

(SELECT * FROM OldStuff);END;

CREATE TRIGGER AvgNetWorthTriggerAFTER UPDATE OF netWorth ON MovieExecREFERENCING

OLD TABLE AS OldStuff,NEW TABLE AS NewStuff

FOR EACH STATEMENTWHEN (50000 > (SELECT AVG(netWorth) FROM MovieExec))BEGIN

DELETE FROM MovieExecWHERE (Name, address, cert#, netWorth) IN

NewStuff;INSERT INTO MovieExec

(SELECT * FROM OldStuff);END;


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End Examples from sqlzoo.net

For next time: read 8.1-2

m.p. johnson, dbms, stern/nyu, sp20041 c20.0046: database management systems lecture #13 matthew p....

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