Active DatabasesCIS 6711 Triggers: The Problem - Examples from COMPANY Database 1.Limit all salary...

Active Databases CIS 671 1

Triggers: The Problem -Examples from COMPANY Database

1. Limit all salary increases to 50%. 2. Enforce policy that salaries may never decrease.

3. Maintain TotalSalary in DEPARTMENT relation as employees and their salaries change.

4. Inform a supervisor whenever a supervisee’s salary becomes larger than the supervisor’s.

5. All new hires for a given job code get the same starting salary, which is available in the STARTING_PAY table.

EMPLOYEE(Name, SSN, Salary, DNO, SupervisorSSN, JobCode)DEPARTMENT(DNO, TotalSalary, ManagerSSN) STARTING_PAY(JobCode, StartPay)


The Problem:Example from BANK Database

Branch(BranchID, BranchName, BranchCity, Assets)Customer(CustID, CustName, CustStreet, CustCity)

Account(AccountNo, BranchID, Balance)AccountCustomer(AccountNo, CustID)

Loan(LoanNo, BranchID, Balance)LoanCustomer(LoanID, CustID)

6. Branch Assets are maintained as sum(Account.Balance) for each branch.

7. Overdrafts do not produce a negative balance. Instead they are treated as a loan. The account balance is set to 0 and a loan is created for the amount of the overdraft.


The Problem:Example from TEMPERATURE Database

TEMPS City Temp

Denver 25

Columbus 15

Anchorage

-15

EXTREMES City HighTemp

HighDate

LowTemp

LowDate

Denver 100 7/30 -10 1/20

Columbus 95 7/5 2 2/14

Anchorage

87 8/3 -10 2/16

8. Given table of temperatures TEMPS, that is periodically updated, keep the table of extreme temperatures EXTREMES up to date.

Create the EXTREMES table and populate with all the cities in the TEMPS table, setting the other attributes to null.


What is Needed?The Event-Condition-Action Model

(ECA Model)

• Rules (or triggers) with three components:– Event triggering the rule. (insert, delete, update)

• E.g., an employee’s salary changes.

– Condition to determine if rule action should be executed.

• E.g., is new Temp in City higher than HighTemp for that City?

– Action to be taken.• E.g., update the Department’s Total Salary.


What is Needed?The Event-Condition-Action Model

(ECA Model), continued

• Actions may apply before or after the triggering event is executed.

• An SQL statement may change several rows.– Apply action once per SQL statement.– Apply action for each row changed by SQL

statement.


Availability

• Triggers included in SQL 1999 (SQL 3) – Not in earlier standards.

• Included much earlier in most products:– Oracle, Sybase, DB2– As a consequence syntax may differ from the

standard.


The Problem:Examples from COMPANY Database

1. Limit all salary increases to 50%.2. Enforce policy that salaries may never decrease.3. Maintain TotalSalary in DEPARTMENT relation as

employees and their salaries change. (EN, Fig. 23.2 (a))4. Inform a supervisor whenever a supervisee’s salary

becomes larger than the supervisor’s. (EN, Fig. 23.2 (b))5. All new hires for a given job code get the same starting

salary, which is available in the STARTING_PAY table.



1. COMPANY DatabaseLimit all salary increases to 50%

before trigger emp_salary_limit

create trigger emp_salary_limitbefore update of EMPLOYEEfor each row

when (new.Salary > 1.5 * old.Salary)set new.Salary = 1.5 * old.Salary;

EMPLOYEE(Name, SSN, Salary, DNO, SupervisorSSN, JobCode)

“new” refers to the new tuple.

“old” refers to the old tuple.


2. COMPANY Database Enforce policy that salaries may never decrease

before trigger emp_salary_no_decrease

create trigger emp_salary_no_decreasebefore update of EMPLOYEEfor each row

when (new.Salary < old.Salary)begin

log the event;signal error condition;

end

Method depends on DBMS.

EMPLOYEE(Name, SSN, Salary, DNO, SupervisorSSN, JobCode)


5. COMPANY Database: All new hires for a given job code get the same starting salary, which is available in the STARTING_PAY table.

before trigger emp_start_pay

create trigger emp_start_paybefore insert on EMPLOYEEfor each row

set Salary =(select StartPayfrom STARTING_PAYwhere JobCode = new.JobCode)

EMPLOYEE(Name, SSN, Salary, DNO, SupervisorSSN, JobCode)STARTING_PAY(JobCode, StartPay)


7. BANK Database: Overdrafts do not produce a negative balance. Instead they are treated as a loan. The account balance is set to 0 and a loan is created for the amount of the overdraft.

after trigger overdraft_trigger


7. BANK Database: Overdrafts, continued after trigger overdraft_trigger

Branch(BranchID, BranchName, BranchCity, Assets)Customer(CustID, CustName, CustStreet, CustCity)

Account(AccountNo, BranchID, Balance)AccountCustomer(AccountNo, CustID)

Loan(LoanNo, BranchID, Balance)LoanCustomer(LoanID, CustID)

• Insert a new tuple in the Loan relation, using same branch as the account. Make LoanNo the same as the AccountNo and the Balance the amount of the overdraft.

• Insert a new tuple in the LoanCustomer relation relating the new loan to this customer.

• Set the Balance of the Account tuple to 0.


7. BANK Database: Overdrafts, continued after trigger overdraft_trigger

create trigger overdraft_trigger after update on Accountfor each rowwhen new.balance < 0begin

insert into Loan values(new.AccountID, new.BranchID, -new.Balance);

insert into LoanCustomer(select CustID, AccountNofrom AccountCustomer ACwhere new.AccountID = AC.AccountID);

update Accountset Balance = 0where Account.AccountNo = new.AccountNo;

end;


8. TEMPS Database: Create the EXTREMES table and populate with all the cities in the TEMPS table, setting the other attributes to null.

after triggers HighTempUpdate and LowTempUpdate

TEMPS(City, Temp)EXTREMES(City, HighTemp, HighDate, LowTemp, LowDate)

• Need two after triggers– one for updating the high temperature (HighTempUpdate), – the other for updating the low temperature (LowTempUpdate).

• They will be very similar.


8. TEMPS Database: continued. High Tempearture Update

create trigger HighTempUpdateafter update of TEMPSfor each row

when (new.Temp >( select HighTempfrom EXTREMESwhere City= new.City)

or (select HighTemp from EXTREMES

where City = new.City) is null ))update EXTREMES

set HighTemp = new.Temp,HighDate = current date

where City = new.City;


Initially HighTemp

is null.

Normal situation.

Low Temperature similar.


8. TEMPS Database: continued. Other Problems

• Insert a new City into TEMPS.– Must also insert into EXTREMES.– Initial values of HighTemp, HighDate, LowTemp, LowDate must be set.

• Delete a City from TEMPS.– Leave City in EXTREMES.– Delete City from EXTREMES.


8. TEMPS Database: continued.Insert a new City into TEMPS:

Insert City tuple into EXTREMES.Set initial values of HighTemp, HighDate,

LowTemp, LowDate.

create trigger NewCityafter insert of TEMPSfor each row

insert into EXTREMES(City,HighTemp, HighDate,LowTemp, LowDate)

values(new.City, new.Temp, Current Date, new.Temp, Current Date);



8. TEMPS Database: continued.Delete a City from TEMPS:

Delete City tuple from EXTREMES.Add Foreign Key constraint for EXTREMES.

Alter table EXTREMESadd constraint fk

foreign key(City) references TEMPSon delete cascade;



Problems with Use of Triggers

• How to guarantee set of triggers is consistent?• Recursion is allowed.

– How to guarantee termination?

• Tools are still needed to help address these problems.


Triggers: The Problem -9. Another Example from

COMPANY Database

1. Add a new field, Span, to the EMPLOYEE relation. • For each employee, Span is the number of employees supervised.

2. Initialize Span appropriately.3. Keep Span correct as the database changes.


EMPLOYEE(Name, SSN, Salary, DNO, SupervisorSSN, JobCode, Span)


9. COMPANY Database

For each employee, Span is the number of employees supervised.

Idea

• Add the Span attribute to EMPLOYEE.

• Initialize the values of Span based on the current database.

• Create insert, delete and update triggers to keep Span up to date for an employee’s immediate supervisor.

• Create another update trigger to propagate the change in Span up through the rest of the hierarchy.

EMPLOYEE(Name, SSN, Salary, DNO, SupervisorSSN, JobCode, Span)


9. COMPANY DatabaseUpdate Span for the immediate supervisor.

create trigger EmpHire after insert on EMPLOYEE

for each rowupdate EMPLOYEE

set Span = Span + 1where SSN =

new.SupervisorSSN;

EMPLOYEE(Name, SSN, ..., SupervisorSSN,..., Span)

create trigger EmpTransfer after update of SupervisorSSN

on EMPLOYEEfor each row

beginupdate EMPLOYEE

set Span = Span - 1where SSN =

old.SupervisorSSN;update EMPLOYEE

set Span = Span + 1where SSN =

new.SupervisorSSN;end;

create trigger EmpQuit after delete on EMPLOYEE

for each rowupdate EMPLOYEE

set Span = Span - 1where SSN =

old.SupervisorSSN;


9. COMPANY DatabasePropagate Span up the supervisor tree.

EMPLOYEE(Name, SSN, Salary, DNO, SupervisorSSN, JobCode, Span)A 123 456 4B 456 789 8

create trigger EmpPropagate after update of Span on

EMPLOYEEfor each row

update EMPLOYEEset Span = Span +

(new.Span - old.Span) where SSN =

new.SupervisorSSN;

Supervisor

+(new.Span - old.Span)decrease (4 - 5) = -1increase (4 - 3) = +1

new value


10. For each PERSON, record their mother, father and number of descendants.

PERSONS(Name, Mother, Father, NumDescendants)

create trigger NewMother after insert on PERSONS

for each rowupdate PERSONS

set NumDescendants = NumDescendants + 1

where Name = new.Mother;

create trigger NewFather after insert on PERSONS


set NumDescendants = NumDescendants + 1

where Name = new.Father;

Then update the maternal and paternal ancestors.

After insert, update the mother and father.


10. For each PERSON, record their mother, father and number of descendants.

Update the maternal and paternal ancestors.

create trigger MaternalAncestor after update of NumDescendants on PERSONS


set NumDescendants = NumDescendants + new.NumDescendants- old.NumDescendants

where Name = new.Mother;

At nth level of family tree, how many triggers?

PERSONS(Name, Mother, Father, NumDescendants)

create trigger PaternalAncestor after update of NumDescendants on PERSONS

/* Similar. Just replace “Mother” with “Father.” */

Active DatabasesCIS 6711 Triggers: The Problem - Examples from COMPANY Database 1.Limit all salary...

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