1. Triggers

2. Triggers: Motivation
Assertions are powerful, but the DBMS often can’t tell when they need to be checked.
Attribute- and tuple-based checks are checked at known times, but are not powerful.
Triggers let the user decide when to check for any condition.

3. Event-Condition-Action Rules
Another name for “trigger” is event-condition-action (ECA) rule.
Event : typically a type of database modification, e.g., “insert on Sells.”
Condition : Any SQL boolean-valued expression.
Action : Any SQL statements.

4. Example
Using Sells(bar, beer, price) and a unary relation, maintain a list of bars that raise the price of any beer by more than $1.
Let the unary relation be RipoffBars(bar).
CREATE TABLE Sells(
beer VARCHAR(10),
bar VARCHAR(13),
price FLOAT );
CREATE TABLE RipOffBars(
bar VARCHAR(13)

5. The Trigger CREATE OR REPLACE TRIGGER PriceTrig
Event:
only changes
to prices
CREATE OR REPLACE TRIGGER PriceTrig
AFTER UPDATE OF price ON Sells
FOR EACH ROW
WHEN(new.price > old.price )
BEGIN
INSERT INTO RipoffBars
VALUES(:new.bar);
END PriceTrig; /
Remark. This and other trigger examples are in ORACLE syntax which differs slightly from standard SQL syntax.
We need to consider
each price change
Condition:
a raise in price > $1
Updates let us talk about old and new tuples.
When the price change is great enough, add the bar to RipoffBars

6. Options: CREATE TRIGGER
CREATE TRIGGER <name>
Or:
CREATE OR REPLACE TRIGGER <name>
Useful if there is a trigger with that name and you want to modify the trigger.
If creating the trigger gives:
Warning: Trigger created with compilation errors.
Execute:
show errors
to display the errors.

7. Options: The Event AFTER can be BEFORE.
UPDATE ON can be DELETE ON or INSERT ON.
And UPDATE ON can be UPDATE …OF… ON mentioning a particular attribute in relation.

8. Options: FOR EACH ROW
Triggers are either “row-level” or “statement-level.”
FOR EACH ROW indicates row-level; its absence indicates statement-level.
Row level triggers : execute once for each modified tuple.
Statement-level triggers : execute once for an SQL statement, regardless of how many tuples are modified.

9. Row Triggers
In ORACLE
For an update trigger, the old attribute value can be accessed using :old.<column>
and the new attribute value can be accessed using
:new.<column>
For an insert trigger, only :new.<column> can be used.
For a delete trigger only :old.<column> can be used.
In WHEN clause of the trigger use old.<column>, new.<column> (i.e. no colon :)

10. Options: The Condition
Any boolean-valued condition.
Evaluated on the database as it would exist before or after the triggering event, depending on whether BEFORE or AFTER is used.

11. Options: The Action
Surround by BEGIN END.

12. Another Example CREATE TABLE emp ( empno INT, ename VARCHAR(30),
deptno INT,
sal FLOAT,
comm FLOAT );
The following is a before row-level trigger that calculates the commission of every new employee belonging to department 30 that is inserted into the emp table.
CREATE OR REPLACE TRIGGER emp_comm_trig
BEFORE INSERT ON emp
FOR EACH ROW
BEGIN
IF :NEW.deptno = 30 THEN
:NEW.comm := :NEW.sal * .4;
END IF;
END; /

13. Let’s trigger INSERT INTO emp VALUES (9005,'ROBERS',30, 3000,NULL);
INSERT INTO emp VALUES (9006,'ALLEN',30, 4500,NULL);
SELECT * FROM emp WHERE empno IN (9005, 9006);
EMPNO ENAME DEPTNO SAL COMM
9005 ROBERS
9006 ALLEN

14. Trigger Syntax (in ORACLE)
Below is the syntax for creating a trigger in ORACLE (which differs slightly from standard SQL syntax):
CREATE [OR REPLACE] TRIGGER <trigger_name>
{BEFORE|AFTER} {INSERT|DELETE|UPDATE} ON <table_name>
[FOR EACH ROW
[WHEN (<trigger_condition>)]]
BEGIN
<trigger_body>
END <trigger_name>; /
You may specify up to three triggering events using the keyword OR. Here are some examples:
... INSERT ON R ...
... INSERT OR DELETE OR UPDATE ON R ...

15. Restrictions Some restrictions on <trigger_body> include:
You can’t modify the same relation whose modification is the event triggering the trigger.
You can’t modify a relation which is the “parent” of the triggering relation in a foreign-key constraint.

16. Miscellaneous about Triggers
Displaying Trigger Definition Errors
If you get a message Warning: Trigger created with compilation errors. you can see the error messages by typing
SHOW ERRORS TRIGGER <trigger_name>;
Alternatively, you can type, SHOW ERRORS to see the most recent compilation error.
Note that the reported line numbers where the errors occur are not accurate.
Viewing Defined Triggers
To view a list of all defined triggers, use:
SELECT trigger_name
FROM user_triggers;
For seeing the code:
SELECT text
FROM user_source
WHERE name = 'PRICETRIG'
ORDER BY line;
Has to be uppercase.

17. Miscellaneous (Cont’d)
Dropping Triggers
DROP TRIGGER <trigger_name>;
Disabling or Enabling Triggers
ALTER TRIGGER <trigger_name> {DISABLE|ENABLE};

18. Aborting Triggers with Errors
Triggers can often be used to enforce constraints. The WHEN clause or body of the trigger can check for the violation of certain conditions and signal an error accordingly using the ORACLE built-in function RAISE_APPLICATION_ERROR.
The action that activated the trigger (insert, update, or delete) would be aborted. For example, the following trigger enforces the constraint Person.age >= 0:
CREATE TABLE Person (age INT);
CREATE TRIGGER PersonCheckAge
AFTER INSERT OR UPDATE OF age ON Person
FOR EACH ROW
BEGIN
IF (:new.age < 0) THEN
RAISE_APPLICATION_ERROR(-20000, 'no negative age allowed');
END IF;
END; /

19. Aborting Triggers with Errors (Cont’d)
If we attempted to execute the insertion:
INSERT INTO Person VALUES (-3);
we would get the error message:
ERROR at line 1: ORA-20000: no negative age allowed ORA-06512: at "MYNAME.PERSONCHECKAGE", line 3 ORA-04088: error during execution of trigger 'MYNAME.PERSONCHECKAGE'
and nothing would be inserted.
In general, the effects of both the trigger and the triggering statement are rolled back.

20. Statement-Level Trigger
Whenever an insert, update, or delete operation occurs on the emp table, a row is added to the empauditlog table recording the date, user, and action.
First let’s create the empauditlog table:
CREATE TABLE empauditlog (
audit_date DATE,
audit_user VARCHAR2(20),
audit_desc VARCHAR2(20) );

21. Now the trigger CREATE OR REPLACE TRIGGER emp_audit_trig
AFTER INSERT OR UPDATE OR DELETE ON emp
DECLARE
v_action VARCHAR2(20);
BEGIN
IF INSERTING THEN
v_action := 'Added employee(s)';
ELSIF UPDATING THEN
v_action := 'Updated employee(s)';
ELSIF DELETING THEN
v_action := 'Deleted employee(s)';
END IF;
INSERT INTO empauditlog
VALUES (SYSDATE, USER, v_action);
END; /

22. Let’s trigger it…
INSERT INTO emp(empno, ename, deptno) VALUES (9001,'SMITH',50);
INSERT INTO emp(empno, ename, deptno) VALUES (9002,'JONES',50);
UPDATE emp
SET ename = 'SMITH BROWN'
WHERE empno=9001;
DELETE FROM emp WHERE empno IN (9001, 9002);
SELECT TO_CHAR(AUDIT_DATE,'DD-MON-YY HH24:MI:SS') AS "AUDIT DATE",
audit_user, audit_desc
FROM empauditlog
ORDER BY 1 ASC;
AUDIT DATE AUDIT_USER AUDIT_DESC
08-FEB-08 09:43:02 THOMO Added employee(s)
08-FEB-08 09:43:02 THOMO Deleted employee(s)
08-FEB-08 09:43:02 THOMO Updated employee(s)