Lab 9

Triggers- View-Sequence

Eng. Ibraheem Lubbad

The Islamic University of Gaza

Faculty of Engineering

Dept. of Computer Engineering

Database Lab (ECOM 4113)

Triggers:

A trigger is a PL/SQL block or a PL/SQL procedure associated with a table, view, schema,

or database. Executes implicitly whenever a particular event takes place. Oracle

Database automatically executes a trigger when specified conditions occur

Trigger Timing: When should the trigger fire?

BEFORE: Execute the trigger body before the triggering DML event on a table not

on views.

AFTER: Execute the trigger body after the triggering DML event on a table not on

views.

NSTEAD OF: Execute the trigger body instead of the triggering statement. This is

used for views that are not otherwise modifiable, trigger on the nested table

column of the view.

A trigger event: Determines which DML statement causes the trigger to execute.

Types are:

INSERT

UPDATE [OF column]

DELETE

A trigger body (Executable-statements): Is a PL/SQL block or a CALL to a procedure. It

determines what action is performed.

The syntax of trigger

Of CREATE [OR REPLACE ] TRIGGER trigger_name

{BEFORE | AFTER | INSTEAD OF } -- TIMING

{INSERT [OR] | UPDATE [OR] | DELETE} -- EVENT

[OF col_name]

ON table_name

[REFERENCING OLD AS o NEW AS n]

[FOR EACH ROW]

WHEN (condition)

DECLARE

Declaration-statements

BEGIN

Executable-statements

END ;

Type of trigger:

Row trigger (row level)

Statement trigger (table level )

The trigger type determines whether the body executes for each row or only

once for the triggering statement.

A row trigger(row level):

An event is triggered for each row upated, inserted or deleted.

Is not executed if the triggering event does not affect any rows.

Is indicated by specifying the “FOR EACH ROW” clause.

Useful if the code in the trigger action depends on data provided by the

triggering statement or rows that are affected.

*If there are four instructor in department Computer Science, then the trigger fires four times when this statement is entered, because four rows are affected.

CREATE OR REPLACE TRIGGER SALARY_CHANGE

BEFORE UPDATE ON INSTRUCTOR

FOR EACH ROW

WHEN (OLD.SALARY <> NEW.SALARY)

BEGIN

DBMS_OUTPUT.PUT_LINE('OLD SALARY =' || :OLD.SALARY );

DBMS_OUTPUT.PUT_LINE('NEW SALARY =' || :NEW.SALARY );

DBMS_OUTPUT.PUT_LINE('DIFF SALARY =' || SAL_DIFF);

END;

UPDATE INSTRUCTOR

SET SALARY= SALARY+ SALARY*.1

WHERE ID= 10101;

UPDATE INSTRUCTOR SET

SALARY= SALARY+ SALARY*.1

WHERE dept_name= ‘Comp. Sci.’;

Example: write a trigger that is fire when someone is trying to modify an instructor’s

salary. The trigger will insert the values of the old salary and the new salary into an

AUDIT table named “AUDIT_INST”.

1. Create “AUDIT_INST” table.

2. Create trigger “TRACK_SAL_CHANGES”

Create table AUDIT_INST (

INST_ID VARCHAR2(5),

PREV_SAL NUMBER(8,2),

NEW_SAL NUMBER(8,2),

MODFIY_DATE DATE

);

CREATE OR REPLACE TRIGGER TRACK_SALARY_CHANGE

AFTER UPDATE

OF SALARY

ON INSTRUCTOR

REFERENCING OLD AS O NEW AS N

FOR EACH ROW

WHEN (O.SALARY <> N.SALARY)

BEGIN

INSERT INTO AUDIT_INST VALUES(:O.ID,:O.SALARY,:N.SALARY ,SYSDATE);

END;

UPDATE INSTRUCTOR SET SALARY= 100000 WHERE ID= 12121;

UPDATE INSTRUCTOR SET SALARY= 110000 WHERE ID= 12121;

UPDATE INSTRUCTOR SET SALARY= 120000 WHERE ID= 12121;

Example: Create a trigger can be used to keep the tot_cred attribute value, up-to-date

when the grade attribute is updated for a row in the takes table. The trigger is executed

only when the grade attribute is updated from a value that is either null or ’F’, to a

grade that indicates the course is successfully completed

A statement trigger (table level ):

The absence of the FOR EACH ROW option indicates that the trigger fires only once for

each applicable statement, but not separately for each row affected by the statement.

An event is triggered for each sql statement executed

Is the default type of trigger?

Fires once even if no rows are affected at all

Useful if the code in the trigger action does not depend on the data

provided by the triggering statement or the rows affected

In this type we cannot use NEW, OLD

For example, if a DELETE statement deletes several rows from a table, a statement-

level DELETE trigger is fired only once.

CREATE OR REPLACE TRIGGER CREDIT_EARNED

AFTER UPDATE OF GRADE ON TAKES

FOR EACH ROW

WHEN((NEW.GRADE<> 'F' AND NEW.GRADE IS NOT NULL ) AND

(OLD.GRADE= 'F' OR NEW.GRADE IS NULL ))

BEGIN

UPDATE STUDENT

SET TOT_CRED= TOT_CRED+(

SELECT CREDITS FROM COURSE

WHERE COURSE_ID= :NEW.COURSE_ID)

WHERE ID = :NEW.ID;

END;

For more than one type of DML operation can fire a trigger

For Example: [NSERT OR DELETE OR UPDATE ] ON Std_tab), the trigger body can use

the conditional predicates INSERTING, DELETING, and UPDATING to check which type of

statement fire the trigger.

Example: create a (LOGS_STUDENT) to keep track actions on STUDENT table. That

monitor all operation

1. Create “LOGS_STUDENT” table.

2. Create trigger “TRACK_STUDENT_TABLE”

CREATE TABLE LOGS_STUDENT (

USERNAME VARCHAR2(20),

OPERATION_TYPE VARCHAR2(20),

OPERATION_DATE DATE

);

CREATE OR REPLACE TRIGGER TRACK_STUDENT_TABLE

AFTER UPDATE OR DELETE OR INSERT

ON STUDENT

DECLARE

USERNAME VARCHAR2(20);

OPERATION_TYPE VARCHAR2(20);

BEGIN

SELECT USER INTO USERNAME FROM DUAL ;

IF INSERTING THEN

OPERATION_TYPE := 'INSERTING ';

ELSIF UPDATING THEN

OPERATION_TYPE := 'UPDATING ';

ELSE

OPERATION_TYPE:= 'DELETING ';

END IF;

INSERT INTO LOGS_STUDENT VALUES(USERNAME,OPERATION_TYPE,SYSDATE);

END;

Get current user to database

Return true if event is insert

Views:

A views are a stored query, which represent the data of one of more tables. Views can

be based on actual tables or another view.

Whatever DML operations you performed on a view they actually affect the base table

of the view. You can treat views same as any other table. You can Query, Insert, Update

and delete from views, just as any other table.

Syntax of Create View

CREATE [OR REPLACE] VIEW view_name AS

SELECT column1, column2.....

FROM table_name WHERE [condition];

[WITH CHECK OPTION][WITH READ ONLY]

Example: Create a view to find all Computer Sciences courses offered in the spring 2010

semester.

Using Views:

We can use the view name to refer to the virtual table that the view generates

Example: Use upper view to find all Computer Sciences courses the spring 2010

semester in the Watson building.

Caution: when you create a view that select an expression, then you have to name this expression with a column alias, in this case select values from the view using alias name:

Example:

Example

CREATE OR REPLACE VIEW CS_SPRING_2010 AS

SELECT COURSE.COURSE_ID, SEC_ID, BUILDING, ROOM_NUMBER, SEMESTER

FROM COURSE JOIN SECTION

ON COURSE.COURSE_ID = SECTION.COURSE_ID

AND COURSE.DEPT_NAME = 'Comp. Sci.'

AND SECTION.SEMESTER = 'Spring' AND SECTION.YEAR = 2010;

Example

SELECT * FROM CS_SPRING_2010

WHERE BUILDING='Watson';

Example

CREATE OR REPLACE VIEW DEPARTMENTS_TOTAL_SALARY AS

SELECT DEPT_NAME, SUM (SALARY) AS "Total Salary"

FROM INSTRUCTOR

group by dept_name;

Then you can select values from the view using alias name

Or we can specify explicitly names for attribute of a view as follows:

Update of a View:

An updatable view is one you can use to insert, update, or delete base table rows.

We can insert values to base table using view, if it has enough value to column on base

table that has not null constraint.

The view Updatable, if the following conditions are all satisfied by the query defining

the view:

The from clause has only one database table.

The select clause contains only attribute names of the table, and does not have

any expressions, aggregates function, or distinct operator.

The query does not have a group by or having clause

The query does not have A subquery in a SELECT list

The query does not have A subquery designated WITH READ ONLY

Example

SELECT DEPT_NAME, "Total Salary"

FROM DEPARTMENTS_TOTAL_SALARY;

Example

CREATE OR REPLACE VIEW DEPARTMENTS_TOTAL_SALARY

(DEPARTMENT,TOTAL_SALARY)AS

SELECT DEPT_NAME, SUM (SALARY) AS "Total Salary"

FROM INSTRUCTOR

group by dept_name;

If you want a join view to be updatable, then all of the following conditions must be

true:

The DML statement must affect only one table underlying the join

For an INSERT / UPDATE statement, the view must not be

created WITH CHECK OPTION,

For example:

We can create view from another view:

Example

CREATE OR REPLACE VIEW FACULTY AS

SELECT ID, NAME ,DEPT_NAME FROM INSTRUCTOR;

Example

view insert into faculty values(50501,'ibraheem','Comp. Sci.')

Use Example

function CREATE OR REPLACE VIEW CS_FACULTY AS

SELECT ID, NAME ,DEPT_NAME FROM FACULTY

WHERE DEPT_NAME='Comp. Sci.';

What about the following statement:

Will row inserted into base table, but logical it’s wrong to insert instructor in Elec. Eng.

Using Comp. Sci. view .

To solve this problem we can use < WITH CHECK OPTION> clause

The insertion is rejected by the database system if row inserted into the view does not

satisfy the view’s where clause

Use faculty view

insert into CS_faculty values(21212,'Edin','Elec. Eng.');

Example

create or replace view CS_faculty as

select id, name ,dept_name from faculty

where dept_name='Comp. Sci.'

with check option ;

WITH READ ONLY clause;

Specify WITH READ ONLY to indicate that the table or view cannot be updated.

Update join view:

We will get error if we try to update multiple table though join view

Example

create or replace view CS_faculty as

select id, name ,dept_name from faculty

where dept_name='Comp. Sci.'

with read only ;

Example

CREATE OR REPLACE VIEW INST_DEPT AS

SELECT ID, NAME ,DEPT_NAME , BUILDING

FROM INSTRUCTOR NATURAL JOIN DEPARTMENT

Example

view INSERT INTO INST_DEPT values('19871','ibraheem','Taylor' );

But still we can use “INST_DEPT” view to insert record in instructor table.

Since any not exist column in view will insert into base table as null value so, cannot

above view to insert department record in base table .

Viewing ‘Views’ in the Data Dictionary

SELECT * FROM USER_VIEWS;

Sequence:

A sequence is a schema object that can generate unique sequential values. These values

are often used for primary and unique keys. You can refer to sequence values in SQL

statements with these pseudocolumns:

CURRVAL: Returns the current value of a sequence

NEXTVAL: Increments the sequence and returns the next value

You must qualify CURRVAL and NEXTVAL with the name of the sequence:

sequence.CURRVAL

sequence.NEXTVAL

To create a sequence, use the following syntax:

By default, the sequence starts with 1 and the increment value for it is 1.

How to Use Sequence Values:

When you create a sequence, you can define its initial value and the increment between

its values. The first reference to NEXTVAL returns the initial value of the sequence.

Subsequent references to NEXTVAL increment the sequence value by the defined

increment and return the new value. Any reference to CURRVAL always returns the

current value of the sequence, which is the value returned by the last reference to

NEXTVAL.

Before you use CURRVAL for a sequence in your session, you must first

initialize the sequence with NEXTVAL.

Within a single SQL statement containing a reference to NEXTVAL, Oracle

increments the sequence once.

Syntax of SEQUENCE

CREATE SEQUENCE SEQUENCE_NAME [START WITH N] [INCREMENT BY M];

CREATE SEQUENCE SEQ;

Example

SELECT SEQ. NEXTVAL FROM DUAL; -- RETURN 1

SELECT SEQ. NEXTVAL FROM DUAL; -- RETURN 2

SELECT SEQ. NEXTVAL FROM DUAL; -- RETURN 3

SELECT SEQ. CURRVAL FROM DUAL; -- RETURN 3

SELECT SEQ. CURRVAL FROM DUAL; -- RETURN 3

SELECT SEQ. NEXTVAL FROM DUAL; -- RETURN 4