Creating Trigger and Maintaining Update Log with PL/SQL

ISYS 650

Introduction to PL/SQL

• Pl/SQL Block-Structured– Variable declaration section– Executable section– Exception-handling section

Variable Declaration

• Variable name: begin with a letter and up to 30 characters.

• Data types:– Character: CHAR(n), VARCHAR2(n)– Number: INTEGER, INT, SMALLINT– DECIMAL(i,j), NUMBER(i,j)– Boolean– Date

Examples

• DECLAREEid CHAR(3);

Ename VARCHAR2(25);

HireDate DATE DEFAULT SYSDATE;

Salary NUMBER(7,2);

TaxRate NUMBER(4,2) :=5.25;

%TYPE

• Use a column’s data type as the data type for the variable:– CustomerName Customer.Cname%TYPE;

Executable Section

• BEGIN– Statements

• END;

• /– “/ “ signals the end of program.

Assignment Statement

• Tax := Salary * TaxRate;

• Arithemetic operators:– +, -, *, /, **

• Concatenation:– ||

• FullName := (FirstName || ‘ ‘ || LastName);

Substitution Variables

• &variableName– Ex. &Salary

• On screen you will see:– Enter value for Salary:

• Select * from Customer• Where CID=‘&CID’;

• Ex1: Get input from screen:– Salary := &Salary;

SELECT … INTO

• SELECT columns separated by commas• INTO variables separated by commas• FROM tablename• WHERE condition;• Ex1:

– SELECT cid, cname INTO custID, customername– FROM customer– WHERE cid = ‘c01’;

• Ex2: Using substituion variable– SELECT cid, cname INTO custID, customername– FROM customer– WHERE cid = ‘&cid’;

Decisions

• IF condition THEN– Statements /* each statement ends with ; */

• END IF;

• IF … THEN … ELSE …END IF;

• IF … THEN ELSIF ……ELSE … END IF;

Loop

• WHILE condition LOOP– Statements

• END LOOP;

• FOR i IN 1 .. 10 LOOP– Statements

• END LOOP

Displaying Output

• SET SERVEROUTPUT ON;• DBMS_OUTPUT.PUT_LINE(string to print);

– DBMS_OUTPUT is a build in package, PUT_LINE is a procedure in that package.

• Ex.– Tax:=Salary * TaxRate;

– DBMS_OUTPUT.PUT_LINE(‘Your tax is: ‘ || Tax);

Script: Caltax.sqldeclare taxRate number(3,2); eid employees.employee_id%type; empSalary employees.salary%type; tax number(7,2);begin taxRate :=&taxRate; select salary into empSalary from employees where employee_id = '&eid';tax := empSalary * taxRate;dbms_output.put_line('Tax is : ' || tax);end;/

Exceptions

• Exceptions are errors.

• Predefined exceptions:– No_Data_Found: Select statement returns no row.

– Too_Many_Rows: Single-Row Select statement returns more than one row.

– Value_Error: Data conversion errors.

Handling Exceptions

• Exception– WHEN exceptionname THEN

– Statements

– WHEN OTHERS THEN– Statements

• Ex: PLSQLException.SQL

DeclareCustName Customer.Cname%Type;

begin select Cname into Custname from customer where cid='&CID';

dbms_output.put_line('Customer name is : ' || Custname);exception when no_data_found then dbms_output.put_line('Customer record not exist');end;/

Functions

CREATE [OR REPLACE] FUNCTION functionname

IS[Variable declarations]BEGIN

StatementsRETURN expression;

END;/

create or replace function emptax(sal employees.salary%type)

return number is

tax number(8,2);

begin

if sal < 2000.0 then

tax:=sal*0.1;

elsif sal <4000.0 then

tax:=sal*0.2;

else

tax:=sal*0.3;

end if;

return tax;

end;

/

Using the User-defined Function with SQL

• Select employee_id,first_name || ' ' || last_name as Ename, empTax(Salary) as Tax

• FROM employees;

Creating Stored Procedure

• A procedure is a named program block:CREATE [OR REPLACE] PROCEDURE procedurename

IS[Variable declarations]BEGIN

Statements

END;/

Note: To run a procedure: EXECUTE procedurename

create or replace procedure addCustomer (

custid customer.cid%TYPE,

cname customer.cname%TYPE,

city customer.city%TYPE,

rating customer.rating%TYPE)

is

tempid customer.cid%TYPE;

begin

select cid into tempid from customer where cid=custid;

dbms_output.put_line('Customer record already exist');

exception

when no_data_found then

insert into customer values(custid,cname,city,rating);

end;

/

Note: To run a stored procedure: execute addcustomer('C12','Mia','LA','C');

Create a procedure to Issue Refresh Command

create or replace procedure RefreshMVCustasbegin DBMS_MVIEW.REFRESH('MVCustomer');end;/

Note: A stored procedure can be called from an application.

Triggers

• A trigger is a program stored in the database and is called automatically when a triggering event occurs.

• Update events: – Insert, Delete, Update

• BEFORE Insert|Delete|Update ON tablename

• AFTER Insert|Delete|Update ON tablename

• Accessing new value and old value:– :NEW.fieldname

– :OLD.fieldname

• FOR EACH ROW

Demo :New and :Oldcreate or replace trigger changesal

after update on employees

for each row

begin

dbms_output.put_line(:old.Last_name || ' old salary is:' || :old.salary);

dbms_output.put_line('new salary is: ' || :new.salary);

end;

/

Note: EmpSaltrigger.sql

create or replace trigger adddetail

before insert on orderdetail

for each row

declare

stocks product.onhand%type;

begin

select onhand into stocks

from hproduct where pid=:new.pid;

if stocks < :new.qty then

update product

set onhand=0

where pid = :new.pid;

dbms_output.put_line('not enough stock');

else

update product

set onhand=onhand - :new.qty

where pid=:new.pid;

end if;

end;

/ Note: AddDetailTrigger.sql

Master Table and User-Defined Log

Log Maintenance

• Log structure:– Record’s fields + Update Time + Update Flag

• Update Flags: – Deletion: D– Insertion: I– Modification is treated as deletion of the old

record with slightly smaller update time and insert of the new record.

Code for Update (Modification)CREATE OR REPLACE TRIGGER "UpdateCustTrigger" after Update on Customer for each row declare UpdTimeStamp CustLog.updatetime%type; begin UpdTimeStamp:=sysdate-0.00005; insert into CustLog values(:old.cid,:old.Cname,:old.City,:old.Rating,UpdTimeStamp,'D'); UpdTimeStamp:=sysdate; insert into CustLog values(:new.cid,:new.Cname,:new.City,:new.Rating,UpdTimeStamp,'I'); end; /

Code for Insertion

CREATE OR REPLACE TRIGGER "InsertCustTrigger" after Insert on Customer for each row declare UpdTimeStamp CustLog.updtime%type; begin UpdTimeStamp:=sysdate; insert into CustLog values(:new.cid,:new.Cname,:new.City,:new.Rating,UpdTimeStamp,'I'); end; /

Code for Deletion

CREATE OR REPLACE TRIGGER "DeleteCustTrigger" after Delete on Customer for each row declare UpdTimeStamp CustLog.updtime%type; begin UpdTimeStamp:=sysdate; insert into CustLog values(:old.cid,:old.Cname,:old.City,:old.Rating,UpdTimeStamp,'D'); end; /

Patterns of Update Flag for an Updated Record

• D, {I, D}: Deletion of an record

• I, {D, I}: A new record

• {D, I}: Modification

• {I, D}: A new record eventually deleted (No impact to MV)

To Select All Deletions from the Log

select * from custlog cwhere updateFlag='D' and updateTime=(select max(updateTime)from custLog where cid=c.cid);

Note: For an updated record, select the last entry (the entry with Max(updateTime)) in the log with a Flag of D

To Select All Insertions from the Log

select * from custlog cwhere updateFlag='I' and updateTime=(select max(updateTime)from custLog where cid=c.cid);

Note: For an updated record, select the last entry (the entry with Max(updateTime)) in the log with a Flag of I

Demo: Remotely call a stored procedure to refresh a MV

Dim strConn As String = "Provider=MSDAORA;Data Source=XE;User ID=dchao;password=???"Dim objConn As New OleDbConnection(strConn)Dim objcomm As New OleDbCommand("RefreshMVCust", objConn)

objcomm.CommandType = CommandType.StoredProcedureobjConn.Open()objcomm.ExecuteNonQuery()objConn.Close()