I would like to use a trigger on a table which will be fired every time a row is inserted, updated, or deleted. Since I would like to do the same things if the row is inserted, updated, or deleted, I would like to know what's happening in the trigger.
How can I know if the row has been deleted? From Using Triggers :. So your trigger would become:. You may need to add logic inside the trigger to cater for code that updates field1 from 'HBP' to something else. How are we doing? Please help us improve Stack Overflow. Take our short survey. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? Collectives on Stack Overflow. A constraint applies to existing data in the table and any statement that manipulates the table. Triggers constrain what a transaction can do. A trigger does not apply to data loaded before the definition of the trigger; therefore, it is not known if all data in a table conforms to the rules established by an associated trigger.
Although triggers can be written to enforce many of the same rules supported by Oracle Database's declarative integrity constraint features, triggers should only be used to enforce complex business rules that cannot be defined using standard integrity constraints.
The declarative integrity constraint features provided with Oracle Database offer the following advantages when compared to constraints defined by triggers:. Centralized integrity checks. All points of data access must adhere to the global set of rules defined by the integrity constraints corresponding to each schema object. Declarative method. Constraints defined using the standard integrity constraint features are much easier to write and are less prone to errors, when compared with comparable constraints defined by triggers.
While most aspects of data integrity can be defined and enforced using declarative integrity constraints, triggers can be used to enforce complex business constraints not definable using declarative integrity constraints. For example, triggers can be used to enforce:. There are many cases where referential integrity can be enforced using triggers. Note, however, you should only use triggers when there is no declarative support for the action you are performing.
The following sections provide examples of the triggers necessary to enforce referential integrity. This operation is necessary to maintain concurrency as the rows are being processed. This exception can be removed if this trigger is used alone. Also, this trigger does not allow triggers to cycle such as, A fires B fires A.
You should carefully test any triggers that require error trapping to succeed to ensure that they always work properly in your environment. Triggers can enforce integrity rules other than referential integrity. For example, this trigger performs a complex check before allowing the triggering statement to run.
Triggers are commonly used to enforce complex security authorizations for table data. Only use triggers to enforce complex security authorizations that cannot be defined using the database security features provided with Oracle Database. Oracle Database Security Guide for details on database security features.
Triggers are very useful when you want to transparently perform a related change in the database following certain events. This type of trigger is useful to force values in specific columns that depend on the values of other columns in the same row. The following example illustrates how a trigger can be used to derive new column values for a table whenever a row is inserted or updated. Views are an excellent mechanism to provide logical windows over table data.
However, when the view query gets complex, the system implicitly cannot translate the DML on the view into those on the underlying tables. These triggers can be defined over views, and they fire instead of the actual DML. Consider a library system where books are arranged under their respective titles. The library consists of a collection of book type objects. The following example explains the schema. You can define a complex view over these tables to create a logical view of the library with sections and a collection of books in each section.
Similarly, you can also define triggers on the nested table booklist to handle modification of the nested table element. System triggers can be used to set application context. Application context is a relatively new feature that enhances your ability to implement fine-grained access control.
Application context is a secure session cache, and it can be used to store session-specific attributes. The trigger setexpensectx ensures that the context is set for every user. Oracle Database's system event publication lets applications subscribe to database events, just like they subscribe to messages from other applications. Oracle Database's system events publication framework includes the following features:. By creating a trigger, you can specify a procedure that runs when an event occurs.
This feature is integrated with the Advanced Queueing engine. When events are detected by the server, the trigger mechanism executes the action specified in the trigger. Only system-defined database events can be detected this way.
You cannot define your own event conditions. When an event occurs, all triggers that are enabled on that event are fired, with some exceptions:. More than one trigger can be created on an object. When an event fires more than one trigger, the order is not defined and you should not rely on the triggers being fired in a particular order. When an event is published, certain runtime context and attributes, as specified in the parameter list, are passed to the callout procedure.
A set of functions called event attribute functions are provided. For each system event supported, event-specific attributes are identified and predefined for the event. You can choose the parameter list to be any of these attributes, along with other simple expressions. For callouts, these are passed as IN arguments. Return status from publication callout functions for all events are ignored. Traditionally, triggers execute as the definer of the trigger.
The trigger action of an event is executed as the definer of the action as the definer of the package or function in callouts, or as owner of the trigger in queues. Because the owner of the trigger must have EXECUTE privileges on the underlying queues, packages, or procedure, this behavior is consistent. Do not define triggers that duplicate features already built into Oracle Database. For example, do not define triggers to reject bad data if you can do the same checking through declarative integrity constraints.
Limit the size of triggers. Use triggers only for centralized, global operations that should be fired for the triggering statement, regardless of which user or database application issues the statement. Do not create recursive triggers. Triggers are implicitly fired by Oracle when a triggering event occurs, no matter which user is connected or which application is being used.
Figure shows a database application with some SQL statements that implicitly fire several triggers stored in the database. Notice that the database stores triggers separately from their associated tables.
A trigger can also call out to a C procedure, which is useful for computationally intensive operations. Triggers supplement the standard capabilities of Oracle to provide a highly customized database management system. For example, a trigger can restrict DML operations against a table to those issued during regular business hours.
You can also use triggers to:. Publish information about database events, user events, and SQL statements to subscribing applications. Although triggers are useful for customizing a database, use them only when necessary. Excessive use of triggers can result in complex interdependencies, which can be difficult to maintain in a large application.
For example, when a trigger fires, a SQL statement within its trigger action potentially can fire other triggers, resulting in cascading triggers.
This can produce unintended effects. Figure illustrates cascading triggers. You can use both triggers and integrity constraints to define and enforce any type of integrity rule. However, Oracle strongly recommends that you use triggers to constrain data input only in the following situations:.
To enforce referential integrity when child and parent tables are on different nodes of a distributed database. When a required referential integrity rule cannot be enforced using the following integrity constraints:.
Figure represents each of these parts of a trigger and is not meant to show exact syntax. The sections that follow explain each part of a trigger in greater detail. A triggering event or statement is the SQL statement, database event, or user event that causes a trigger to fire.
A triggering event can be one or more of the following:. When the triggering event is an UPDATE statement, you can include a column list to identify which columns must be updated to fire the trigger. When multiple types of SQL statements can fire a trigger, you can use conditional predicates to detect the type of triggering statement. In this way, you can create a single trigger that runs different code based on the type of statement that fires the trigger.
A trigger restriction specifies a Boolean expression that must be true for the trigger to fire. The trigger action is not run if the trigger restriction evaluates to false or unknown.
In the example, the trigger restriction is:. Consequently, the trigger does not fire unless the number of available parts is less than a present reorder amount. If the triggers are row triggers, the statements in a trigger action have access to column values of the row being processed by the trigger.
Correlation names provide access to the old and new values for each column. Row Triggers and Statement Triggers. When you define a trigger, you can specify the number of times the trigger action is to be run:. Once for every row affected by the triggering statement, such as a trigger fired by an UPDATE statement that updates many rows. A row trigger is fired each time the table is affected by the triggering statement.
If a triggering statement affects no rows, a row trigger is not run. Row triggers are useful if the code in the trigger action depends on data provided by the triggering statement or rows that are affected. For example, Figure illustrates a row trigger that uses the values of each row affected by the triggering statement. A statement trigger is fired once on behalf of the triggering statement, regardless of the number of rows in the table that the triggering statement affects, even if no rows are affected.
Statement triggers are useful if the code in the trigger action does not depend on the data provided by the triggering statement or the rows affected. For example, use a statement trigger to:. When defining a trigger, you can specify the trigger timing — whether the trigger action is to be run before or after the triggering statement. 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.
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