Syntax:

DROP TRIGGER trigger_name;

Example:

CREATE OR REPLACE TRIGGER EMP_MONDAY

BEFORE INSERT OR UPDATE OR DELETE ON EMP

BEGIN

IF RTRIM(UPPER(TO_CHAR(SYSDATE,'DAY')))='MONDAY' THEN

RAISE_APPLICATION_ERROR
(-20022,'NO OPERARTION CAN BE PERFORMED ON MONDAY');

END IF;

END;

Dropping Trigger

DROP TRIGGER REC_CHECK;

We can disable / enable the trigger by the following syntax:

ALTER TRIGGER <trigger name> DISABLE / ENABLE

Example

CREATE OR REPLACE TRIGGER UPDATE_CHECK BEFORE UPDATE ON EMP
FOR EACH ROW

BEGIN

IF :NEW.SAL<:OLD.SAL THEN

RAISE_APPLICATION_ERROR(-20000,'NEW SALARY
CANNOT BE LESS THAN OLD SALARY');

END IF;

END;

Enabling Trigger

SQL>ALTER TRIGGER UPDATE_CHECK DISABLE;

Trigger Disabled;


Disabling Trigger

SQL>ALTER TRIGGER UPDATE_CHECK ENABLE;

Trigger Enabled;

Syntax:
RAISE_APPLICATION_ERROR (error_number, message);

Here:
error_number
It is a negative integer in the range-20000 to –20999

Message
It is a character string up to 2048 bytes in length

This procedure terminates procedure execution, rolls back any effects of the procedure, and returns a user-specified error number and message. Following example makes use of the RAISE_APPLICAION_ERROR.

To Create a trigger so that no operation can be performed on emp table
on Monday.

Solution:

CREATE OR REPLACE TRIGGER EMP_MONDAY

BEFORE INSERT OR UPDATE OR DELETE ON EMP

BEGIN

IF RTRIM(UPPER(TO_CHAR(SYSDATE,'DAY')))='MONDAY' THEN

RAISE_APPLICATION_ERROR
(-20022,'NO OPERARTION CAN BE PERFORMED ON MONDAY');

END IF;

END;

• Row Level Triggers
• Statement Level Triggers
  
• Before Triggers
  
• After Triggers

Here is a brief description about above triggers:

Row Level Triggers

Row level triggers execute once for each row in a transaction. The commands of row level triggers are executed on all rows that are affected by the command that enables the trigger. For example, if an UPDATE statement updates multiple rows of a table, a row trigger is fired once for each row affected by the UPDATE statement. If the triggering statement affects no rows, the trigger is not executed at all. Row level triggers are created using the FOR EACH ROW clause in the CREATE TRIGGER command.

Statement Level Triggers

Statement level triggers are triggered only once for each transaction. For example when an UPDATE command update 15 rows, the commands contained in the trigger are executed only once, and not with every processed row. Statement level trigger are the default types of trigger created via the CREATE TRIGGER command.

Before  Trigger

Since triggers are executed by events, they may be set to occur immediately before or after those events. When a trigger is defined, you can specify whether the trigger must occur before or after the triggering event i.e. INSERT, UPDATE, or DELETE commands.

BEFORE trigger execute the trigger action before the triggering statement. These types of triggers are commonly used in the following situation:

• BEFORE triggers are used when the trigger action should determine whether or not the triggering statement should be allowed to complete. By using a BEFORE trigger, you can eliminate unnecessary processing of the triggering statement. For example: To prevent deletion on Sunday, for this we have to use Statement level before trigger on DELETE statement.
• BEFORE triggers are used to derive specific column values before completing a triggering INSERT or UPDATE statement.

After Trigger

AFTER trigger executes the trigger action after the triggering statement is executed. AFTER triggers are used when you want the triggering statement to complete before executing the trigger action.

For example: To perform cascade delete operation, it means that user delete the record fro one table, but the corresponding records in other tables are delete automatically by a trigger which fired after the execution of delete statement issued by the user.

When combining the different types of triggering actions, there are mainly 12 possible valid trigger types available to us. The possible configurations are:

• BEFORE INSERT row
• BEFORE INSERT statement
• AFTER INSERT row
• AFTER INSERT statement
• BEFORE UPDATE row
• BEFORE UPDATE statement
• AFTER UPDATE Row
• AFTER UPDATE statement
• BEFORE DELETE row
• BEFORE DELETE statement
• AFTER DELETE row
• AFTER DELETE statement

Instead of Trigger

These Triggers are Defined on a View rather than a table.You can use INSTEAD OF TRIGGER to tell oracle what to do instead of  performing actions that invoked the  trigger.

A trigger is automatically executed without any action required by the user. A stored procedure on other hand needs to be explicitly invoked.This is the main difference between a trigger and a stored procedure.

Parts of a Trigger

A database trigger has three parts:

• Triggering event or Statement.
• Trigger constraint (Optinal).
• Trigger action.

Triggering Event or Statement

A triggering event or statement is the SQL statement that causes a trigger
to be fired. A triggering event can be an INSERT, UPDATE, or
DELETE statement for a specific table.

Trigger Constraint or Restriction

A trigger restriction specifies a Boolean (logical) expression that must
be TRUE for the trigger to fire. The trigger action s not executed if the
trigger restriction evaluates to FALSE. A trigger restriction is an option
available for triggers that are fired for each row. Its function is to
conditionally control the execution of a trigger. A trigger restriction is
specified using a WHEN clause. It is an optional part of trigger.

Trigger Action

A trigger action is the procedure (PL/SQL block) that contains the
SQL statements and PL/SQL code to be executed when a triggering
statement is issued and the trigger restriction evaluates to TRUE.

Syntax For Creating Trigger

CREATE [OR REPLACE] TRIGGER trigger_name
{BEFORE|AFTER}
{|DELETE|[OR]INSERT|[OR]UPDATE [OF column_name...]
ON table_name

[REFERENCING {OLD as old,NEW as new}]
[FOR EACH ROW[WHEN condition]

DECLARE

Variable decalration;
Constant declaration;

BEGIN

PL/SQL subprogram body;

[EXCEPTION
exception PL/SQL block;]
END;

Here:

REFERENCING: Specified correlation names. The user could use the Correlation names in the PL/SQL block and WHEN clause of a row trigger to refer specifically to old and new values of the current row. The default correlation names are OLD and NEW. If the row is associated with a table named OLD or NEW, this clause can be used to specify different names to avoid confusion between the table name and the correlation name.

WHEN: Specifies the trigger restriction. This condition has to be satisfied to fire the trigger. This condition can be specified for the ROW TRIGGER.

Use of Database Triggers

Database triggers can be used for the following purposes:

• To derive column values automatically.
• To enforce complex integrity constraints.
• To enforce complex business rules.
• To customize complex security authorizations.
• To maintain replicate tables.
• To audit data modifications.