Describe what is trigger in SQL Server?

 Posted by Tripati_tutu on 10/20/2010 | Category: Sql Server Interview questions | Views: 3894 | Points: 40
Answer:

In any database including SQL Server a trigger is a procedure that initiates on INSERT, DELETE or UPDATE actions.

Before SQL Server 2000 Triggers are also used to maintain the referential integrity. We can not execute triggers explicitly. The DBMS automatically fires the trigger when data modification events (INSERT, DELETE or UPDATE) happened in the associated table.

Triggers are same as stored procedures in terms of procedural logic that is stored at the database level. Stored procedures are executed explicitly and triggers are event-drive.


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Comments or Responses

Posted by: Chvrsri on: 12/9/2010 | Points: 10
You mentioned only DML triggers . The triggering concept can be applied on DDL components as well like Create , Alter , Drop. But while accessing these we have to be very cautious.
Posted by: Muhsinathk on: 9/13/2012 | Points: 10


A trigger is a pl/sql block structure which is fired when a DML statements like Insert, Delete, Update is executed on a database table. A trigger is triggered automatically when an associated DML statement is executed.

Syntax of Triggers

The Syntax for creating a trigger is:

CREATE [OR REPLACE ] TRIGGER trigger_name

{BEFORE | AFTER | INSTEAD OF }

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

[OF col_name]

ON table_name

[REFERENCING OLD AS o NEW AS n]

[FOR EACH ROW]

WHEN (condition)

BEGIN

--- sql statements

END;

SQL Server 2000 has many types of triggers:

1. After Trigger
2. Multiple After Triggers
3.Instead Of Triggers
4. Mixing Triggers Type


















Posted by: Muhsinathk on: 9/13/2012 | Points: 10

Differences between storedprocedures and triggers

1. When you create a trigger you have to identify event and action of your trigger but when you create s.p you don't identify event and action

2.Trigger is run automatically if the event is occured but s.p don't run automatically but you have to run it manually

3. Within a trigger you can call specific s.p but within a sp you cannot call atrigger

4.Trigger execute implicitly whereas store procedure execute via procedure call from another block.

5.We can call a stored procedure from front end (.asp files, .aspx files, .ascx files etc.) but we can't call a trigger from these files.

6. Stored procedure can take the input parameters, but we can't pass the parameters as an input to a trigger.



We can create a trigger to update the 'product_price_history' table when the price of the product is updated in the 'product' table.

1) Create the 'product' table and 'product_price_history' table

CREATE TABLE product_price_history

(product_id number(5),

product_name varchar2(32),

supplier_name varchar2(32),

unit_price number(7,2) );


CREATE TABLE product

(product_id number(5),

product_name varchar2(32),

supplier_name varchar2(32),

unit_price number(7,2) );

2) Create the price_history_trigger and execute it.

CREATE or REPLACE TRIGGER price_history_trigger

BEFORE UPDATE OF unit_price

ON product

FOR EACH ROW

BEGIN

INSERT INTO product_price_history

VALUES

(:old.product_id,

:old.product_name,

:old.supplier_name,

:old.unit_price);

END;



3) Lets update the price of a product.

UPDATE PRODUCT SET unit_price = 800 WHERE product_id = 100

Once the above update query is executed, the trigger fires and updates the 'product_price_history' table.

4)If you ROLLBACK the transaction before committing to the database, the data inserted to the table is also rolled back.

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