Why trigger is used

This type of trigger is commonly used in the following situations:. When the trigger action determines whether the triggering statement should be allowed to complete. Using a BEFORE trigger for this purpose, you can eliminate unnecessary processing of the triggering statement and its eventual rollback in cases where an exception is raised in the trigger action.

Before modifying each row affected by the triggering statement and before checking appropriate integrity constraints, the trigger action is run, if the trigger restriction was not violated. AFTER statement trigger. After executing the triggering statement and applying any deferred integrity constraints, the trigger action is run.

AFTER row trigger. After modifying each row affected by the triggering statement and possibly applying appropriate integrity constraints, the trigger action is run for the current row provided the trigger restriction was not violated. You can have multiple triggers of the same type for the same statement for any given table.

Multiple triggers of the same type permit modular installation of applications that have triggers on the same tables. Deleting a row in a view could either mean deleting it from the base table or updating some values so that it is no longer selected by the view.

Inserting a row in a view could either mean inserting a new row into the base table or updating an existing row so that it is projected by the view. Updating a column in a view that involves joins might change the semantics of other columns that are not projected by the view. Object views present additional problems.

This operation inevitably involves joins, but modifying joins is inherently ambiguous. As a result of these ambiguities, there are many restrictions on which views are modifiable.

A view is inherently modifiable if data can be inserted, updated, or deleted without using INSTEAD OF triggers and if it conforms to the restrictions listed as follows.

Even if the view is inherently modifiable, you might want to perform validations on the values being inserted, updated or deleted. Here the trigger code performs the validation on the rows being modified and if valid, propagate the changes to the underlying tables.

To modify an object materialized by an object view in the client-side object cache and flush it back to the persistent store, you must specify INSTEAD OF triggers, unless the object view is inherently modifiable. However, it is not necessary to define these triggers for just pinning and reading the view object in the object cache. Oracle Call Interface Programmer's Guide. If the view query contains any of the following constructs, the view is not inherently modifiable and you therefore cannot perform inserts, updates, or deletes on the view:.

If a view contains pseudocolumns or expressions, you can only update the view with an UPDATE statement that does not refer to any of the pseudocolumns or expressions. The triggers on the nested tables fire if a nested table element is updated, inserted, or deleted and handle the actual modifications to the underlying tables. You can use triggers to publish information about database events to subscribers. Applications can subscribe to database events just as they subscribe to messages from other applications.

These database events can include:. Triggers on system events can be defined at the database level or schema level.

For example, a database shutdown trigger is defined at the database level:. Triggers on DML statements can be defined on a table or view. A trigger defined at the database level fires for all users, and a trigger defined at the schema or table level fires only when the triggering event involves that schema or table. Event publication uses the publish-subscribe mechanism of Oracle Streams Advanced Queuing.

A queue serves as a message repository for subjects of interest to various subscribers. Each event allows the use of attributes within the trigger text. For example, the database startup and shutdown triggers have attributes for the instance number and the database name, and the logon and logoff triggers have attributes for the user name. You can specify a function with the same name as an attribute when you create a trigger if you want to publish that attribute when the event occurs.

The attribute's value is then passed to the function or payload when the trigger fires. System events that can fire triggers are related to instance startup and shutdown and error messages.

Triggers created on startup and shutdown events have to be associated with the database. Triggers created on error events can be associated with the database or with a schema. Their attributes include the system event, instance number, and database name.

You can use these triggers to make subscribing applications shut down completely when the database shuts down. For abnormal instance shutdown, these triggers cannot be fired. Their attributes include the system event and error number.

The trigger notifies users when a role transition occurs, so that client connections can be processed on the new primary database and applications can continue to run.

DDL triggers can be associated with the database or with a schema. Their attributes include the system event, the type of schema object, and its name. DDL triggers include the following types of triggers:. DML triggers for event publication are associated with a table.

Triggers on DML statements include the following triggers:. Oracle Database Application Developer's Guide - Fundamentals for more information about event publication using triggers on system events and user events.

For enabled triggers, Oracle automatically performs the following actions:. Oracle runs triggers of each type in a planned firing sequence when more than one trigger is fired by a single SQL statement. First, statement level triggers are fired, and then row level triggers are fired.

Oracle performs integrity constraint checking at a set point in time with respect to the different types of triggers and guarantees that triggers cannot compromise integrity constraints. Oracle manages the dependencies among triggers and schema objects referenced in the code of the trigger action. Oracle uses two-phase commit if a trigger updates remote tables in a distributed database.

In our example above, changes made to the WorkOrder table are recorded a TransactionHistory table. The TransactionHistory table is easily referenced and can be incorporated into end user reports. Triggers can be used to inspect all data before a DML action is performed. Triggers can be used to calculate column values.

For instance, for each customer you may wish to maintain a TotalSales column on the customer record. Of course, for this to remain accurate, it would have to be update every time a sales was made. You should avoid using triggers in place of built in features. Kris Wenzel has been working with databases over the past 30 years as a developer, analyst, and DBA. Kris has written hundreds of blog articles and many online courses. He loves helping others learn SQL. I thought about looking into how to help people with a database certification.

Which certification should I focus on helping people pass? Im face problem to know about that im saw many blog but i didnt got my ansewer. Please write about that or please leave comment. The most common triggers are Insert, Update, and Delete triggers.

One possibility is that the triggers may have been used. Whenever a new user data is entered into the website's database the trigger automatically sends a welcome mail to the new user. I hope now you got the basic idea of triggers.

But what exactly are these triggers? So, let's get started. Triggers are the SQL statements that are automatically executed when there is any change in the database. These triggers help in maintaining the integrity of the data by changing the data of the database in a systematic fashion.

Now, we want to create a trigger that will add marks to each new row of the Marks column whenever a new student is inserted to the table. After creating the trigger, we will write the query for inserting a new student in the database.