Database Triggers
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Triggers are stored database procedures that are automatically invoked in response to certain events like data changes. They allow flexible management of data integrity by enforcing business rules. Triggers can be used to log events, gather statistics, modify data when views are updated, enforce referential integrity across nodes, publish database events, prevent operations during certain hours, and enforce complex integrity rules that cannot be defined with constraints alone. Unlike stored procedures, triggers are not explicitly invoked but rather automatically fire in response to triggering events like data modifications.
Database Triggers
- 1. Created by : Aliya Saldanha
- 2. What is a trigger? Trigger is like a procedure that is automatically invoked by the DBMS in response to specified changes to data base Trigger is like a ‘Daemon that monitors a data base, and is executed when the data base is modified in a way that matches the event specification A data base that has a set of associated triggers is called an active data base
- 3. Unlike a stored procedure, you can enable and disable a trigger, but you cannot explicitly invoke it. While a trigger is enabled, the database automatically invokes it—that is, the trigger fires—whenever its triggering event occurs. While a trigger is disabled, it does not fire.
- 4. You create a trigger with the CREATE TRIGGER statement. You specify the triggering event in terms of triggering statements and the item on which they act. The trigger is said to be created on or defined on the item, which is either a table, a view, a schema, or the database. You also specify the timing point, which determines whether the trigger fires before or after the triggering statement runs and whether it fires for each row that the triggering statement affects. By default, a trigger is created in the enabled state.
- 5. If the trigger is created on a table or view, then the triggering event is composed of DML statements, and the trigger is called a DML trigger. If the trigger is created on a schema or the database, then the triggering event is composed of either DDL or database operation statements, and the trigger is called a system trigger.
- 6. Just like with procedures and functions, creating triggers requires certain privileges which are not part of the default privilege set. If you cannot create triggers from these notes because of permissions, you (or the admin) has to GRANT CREATE TRIGGER privilege on your username. For example, to allow user ‘alex’ to create triggers, I may do something like this: GRANT CREATE TRIGGER TO alex; Note that if you are accessing a public Oracle server you must ask the admin to setup these things for you
- 8. Event A change to data base that activates the trigger • Restriction A trigger restriction specifies a Boolean (logical) expression that must be TRUE for the trigger to fire • Action A procedure that is executed when the trigger is activated. Similar to stored procedures, a trigger action can contain PL/SQL statements
- 9. Row Triggers 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 executed at all. • Statement Triggers 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)
- 10. Before Trigger Execute the trigger action before the triggering statement. Eliminate unnecessary processing of the triggering statement. • After Trigger AFTER triggers are used when you want the triggering statement to complete before executing the trigger action
- 12. CREATE or REPLACE TRIGGER cs348 after INSERT ON weatherforecast FOR EACH ROW WHEN (:new.temp>= 60) BEGIN DBMS_OUTPUT.PUT_LINE(‘NICE WEATHER’); END cs348 / Show error;
- 13. Update table weatherforcast T1 Fired Before Update on weatherforcast For each row Begin Insert into weatherforcast2 values (.. , ..); END; T2 Fired Before Update on weatherforcast 2 For each row Begin Insert into weatherforcast3 values (.. , ..); END;
- 15. CREATE TABLE PERSON ( ID INT, NAME VARCHAR(30), DOB DATE, PRIMARY KEY(ID) ); The above creates a PERSON table with an ID, a NAME and a DOB columns (fields). Also, let’s not forget to setup: SET SERVEROUTPUT ON;
- 16. CREATE OR REPLACE TRIGGER PERSON_INSERT_BEFORE BEFORE INSERT ON PERSON FOR EACH ROW BEGIN DBMS_OUTPUT.PUT_LINE(’BEFORE INSERT OF ’ || :NEW.NAME); END;
- 17. The single INSERT statement fires the trigger. When we run it, we get the print out of ’BEFORE INSERT OF JOHN DOE’. Ie: SQL> INSERT INTO PERSON(ID,NAME,DOB) VALUES (1,’JOHN DOE’,SYSDATE); BEFORE INSERT OF JOHN DOE 1 row created.
- 18. CREATE OR REPLACE TRIGGER PERSON_INSERT_AFTER AFTER INSERT ON PERSON FOR EACH ROW BEGIN DBMS_OUTPUT.PUT_LINE(’AFTER INSERT OF ’ || :NEW.NAME); END;
- 19. INSERT INTO PERSON(ID,NAME,DOB) VALUES (2,’JANE DOE’,SYSDATE); SQL> INSERT INTO PERSON(ID,NAME,DOB) VALUES (2,’JANE DOE’,SYSDATE); BEFORE INSERT OF JANE DOE AFTER INSERT OF JANE DOE 1 row created. Notice that both triggers have fired. One before the INSERT the other one after
- 20. ItemId quantity customerid unitprice 123 1 224 ItemId unitprice 123 $440 ItemId quantity customerid unitprice 123 1 224 440 • What are the uses of triggers? Flexible Management of integrity Trigger Fired Log generation to support auditing & security Prevent invalid transactions
- 21. Automatically generate virtual column values Log events Gather statistics on table access Modify table data when DML statements are issued against views Enforce referential integrity when child and parent tables are on different nodes of a distributed database Publish information about database events, user events, and SQL statements to subscribing applications Prevent DML operations on a table after regular business hours Prevent invalid transactions Enforce complex business or referential integrity rules that you cannot define with constraints USES Advantages