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Constraints and Triggers

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Constraints

Constraint is a relationship among data elements that the DBMS is required to enforce.

Types of Constraint:

Primary key

Foreign key

Attribute-based constraints

Tuple-based constraints

Assertions

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Enforcing Foreign-Key

Constraints -1

Beers(name, manf)

Sells(bar, beer, price)

If there is a foreign-key constraint from attributes of relation R (e.g. Sells) to a key of relation S (e.g. Beers), 2 violations are possible:

1. An insert or update to R (e.g. Sells) introduces values not found in S (e.g. Beers).

Example: An insert or update to Sells that introduces a nonexistent beer

Must be rejected

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Enforcing Foreign-Key

Constraints -2

2. A deletion or update to S (e.g. Beers) causes some tuples of R (e.g. Sells) to dangle

Example: A deletion or update to Beers that removes a beer value found in some tuples of Sells

Can be handled in 3 ways:

1. Default : Reject the modification

2. Cascade : Make the same changes in Sells

E.g.: Delete the “Bud” tuple from Beers and then delete all tuples from Sells that have beer = “Bud”

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Enforcing Foreign-Key

Constraints -3

3. Set NULL : Change the beer to NULL

E.g.: Delete the “Bud” tuple from Beers and

change all tuples of Sells that have beer = “Bud”

to have beer = NULL

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Choosing a Policy

We may choose policies SET NULL or CASCADE by

using:

ON [UPDATE, DELETE][SET NULL CASCADE]

Otherwise, the default (Reject) is used

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Example

CREATE TABLE Sells (

bar CHAR(20),

beer CHAR(20),

price REAL,

FOREIGN KEY(beer)

REFERENCES Beers(name)

ON DELETE SET NULL

ON UPDATE CASCADE

);

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Attribute-Based Checks

Attribute-based checks are constraints on the value

of a particular attribute

Are checked only when a value for that

attribute is inserted or updated

CREATE TABLE Sells ( bar CHAR(20), beer CHAR(20) CHECK ( beer IN (SELECT name FROM Beers)), price REAL CHECK ( price <= 5.00 ) );

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Tuple-Based Checks

Constraint is added as a relation-schema

element

The condition may refer to any attribute of the

current relation

Checked on insert or update only

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Example

Only Mary‟s Bar can sell beer for more than $5:

CREATE TABLE Sells (

bar CHAR(20),

beer CHAR(20),

price REAL,

CHECK (bar = “Mary’s Bar” OR

price <= 5.00)

);

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Assertions

Assertions are database-schema elements, like relations or views.

CREATE ASSERTION <name>

CHECK ( <condition> );

Condition may refer to any relation or attribute in the database schema

In principle, assertions are checked after every modification to any relation of the database

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Example

In Sells(bar, beer, price), no bar may charge an average of more than $5.

CREATE ASSERTION NoRipOffBars CHECK (

NOT EXISTS (

SELECT bar FROM Sells

GROUP BY bar

HAVING AVG(price) > 5.0

));

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Exercise

Consider the following database schema

supplier(supplier#, sname, rating, city)

parts(part#, pname, color, weight)

projects(project#, pjname, city)

spj(supplier#, part#, project#, qty)

Express the constraint: “No project can use more

than 100 units of part P65 ” in SQL

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Solution

spj(supplier#, part#, project#, qty)

No project can use more than 100 units of part P65

CREATE ASSERTION parts-constraint CHECK

(NOT EXISTS (SELECT project#, SUM(qty)

FROM spj

WHERE part#=„P65‟

GROUP BY project#

HAVING SUM(qty)>100))

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Triggers: Motivation

Assertions are powerful, but the DBMS

often can‟t tell when they need to be

checked

Attribute- and tuple-based checks are

checked at known times, but are not

powerful

Triggers let the user decide when to check

for a powerful condition

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Event-Condition-Action Rules

Trigger is also called event-condition-action (ECA) rule

Event

Is a type of database modification that fires the trigger

Condition

Is the condition to perform the action

Can be any SQL boolean-valued expression

Action

Can be any SQL statements

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Preliminary Example

Write a trigger so that, whenever there is

an insertion into Sells(bar, beer, price) with

unknown beers, add that beer name to

Beers table, with manufacturer set to

NULL.

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Example

CREATE TRIGGER BeerTrigger

AFTER INSERT ON Sells

REFERENCING NEW ROW AS NewTuple

FOR EACH ROW

WHEN (NewTuple.beer NOT IN

(SELECT name FROM Beers))

INSERT INTO Beers(name)

VALUES(NewTuple.beer);

The event

The condition

The action

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Options: CREATE TRIGGER

CREATE TRIGGER <name>

Option:

CREATE OR REPLACE TRIGGER <name>

Useful if there is a trigger with that name and you

want to modify the trigger

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Options: The Event

AFTER INSERT ON <Attribute>

AFTER can be BEFORE

Example: BEFORE INSERT ON Sells

INSERT can be DELETE or UPDATE.

And UPDATE can be UPDATE … OF … for a

particular attribute

Example: AFTER UPDATE OF price ON Sells

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Options: REFERENCING

AFTER INSERT ON Sells

REFERENCING NEW ROW AS NewTuple

INSERT statements imply a new row or new table

(i.e., a set of inserted tuples)

DELETE implies an old row or old table

UPDATE implies both

[NEW OLD][ROW TABLE] AS <name>

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Options: FOR EACH ROW

FOR EACH ROW indicates row-level;

Its absence indicates statement-level

Row Level Triggers:

Execute once for each modified tuple

Statement-Level Triggers:

Execute once for an SQL statement,

regardless of how many tuples are modified

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Options: The Condition

Any boolean-valued condition is

appropriate

It is evaluated before or after the triggering

event, depending on whether BEFORE or

AFTER is used in the event

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Options: The Action

There can be one or more SQL statement in

the action

Surround by BEGIN . . . END if there is more than

one

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Example

Sells(bar, beer, price)

RipOffBars(bar)

Write a trigger so that any bar that raises the

price of any beer by more than $1 will be

inserted into RipOffBars relation

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The Trigger

CREATE TRIGGER PriceTrigger

AFTER UPDATE OF price ON Sells

REFERENCING

OLD ROW AS oldRow

NEW ROW AS newRow

FOR EACH ROW

WHEN(newRow.price > oldRow.price + 1.00)

INSERT INTO RipOffBars

VALUES(newRow.bar);

The event – When there is change to the price

Since it’s Updates, we have old and new tuples

We need to consider each price change

Condition: a raise in price > $1

When the price change is great enough, add the bar to RipoffBars

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Exercise

Given the following relational schema:

Employee(ID, name, address, position)

CompanyStats(numEmployee, numProduct,

revenue)

Create two triggers that will result in the automatic

tracking of the number of employees a company

manages.

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Solution - 1

The first trigger increments the number of employees each time a new person is hired

CREATE TRIGGER Employee_Hired

AFTER INSERT ON Employee

FOR EACH ROW

UPDATE CompanyStats

SET numEmployee = numEmployee + 1

Each time a new row is inserted into Employee table, numEmployee in CompanyStats table will be incremented by 1

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Solution - 2

The second trigger decrements the number of employees each time an employee leaves the company

CREATE TRIGGER Employee_Leaves

AFTER DELETE ON Employee

FOR EACH ROW

UPDATE CompanyStats

SET numEmployee = numEmployee - 1

Each time a row is deleted from Employee table, numEmployee in CompanyStats table will be decremented by 1

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Exercise

We have a Student table that contains the student ID (SID), the GPA of student, etc.

For each new student with GPA > 4.0, he/she will be automatically be enrolled in the honor seminar course (CS123)

The Enroll table is defined as:

Enroll (SID, courseNumber)

Write a trigger to automatically enroll the student in honor seminar

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Solution

CREATE TRIGGER HonorSeminarEnrollment

AFTER INSERT ON Student

REFERENCING NEW ROW AS newStudent

FOR EACH ROW

WHEN (newStudent.GPA > 4.0)

Insert Into Enroll

Values (newStudent.SID, „CS123‟);