ISYS 365 - Triggers

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ISYS 365 - Triggers

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Agenda Triggers

Review Correlation identifiers (pseudo records) Restrictions on triggers Trigger usage Mutating tables Enabling and disabling triggers Data Dictionary

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Triggers: SyntaxCREATE OR REPLACE TRIGGER trigger_Name

{BEFORE|AFTER} {INSERT|UPDATE| DELETE}[OF column_name] ON table_name[FOR EACH ROW][WHEN trigger_condition]

DECLARE -- variable declaration goes here

BEGINIF INSERTING THEN -- statements hereELSIF UPDATING THEN -- statements hereELSIF DELETING THEN -- statements hereEND IF;

END [trigger_Name];

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Recommended Naming Convention

Table_Name_[A|B][I|U|D][S|R] [A|B] AFTER or BEFORE [I|U|D] INSERT, UPDATE, DELETE [S|R] statement-level or row-level

Example: Employee_AIUS Employee_BUR Department_BIUDR

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Triggers: Correlation Identifiers Correlation identifiers

Row-level triggers can access individual column values

:old and :new (called pseudo records) Syntactically treated as records of type

triggering_table%ROWTYPE Reference fields within the pseudorecords

using dot notation (just like implicit records) :old.salary :new.salary

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Triggers: Correlation Identifiers :old.column_name

Contains the value prior to the change

NULL for INSERT statements :new.column_name

Contains the value after the change NULL for DELETE statements

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Triggers: Correlation Identifiers

Testing :old value against :new value will only work for UPDATE statement

:old.salary <> :new.salary

INSERT has only :new value DELETE has only :old value

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Example of correlation identifiers

/* Log any changes to employee salary where the increase is greater than 10% */

CREATE OR REPLACE TRIGGER employee_BURBEFORE UPDATE ON employeeFOR EACH ROW

WHEN (NEW.amount/OLD.amount > 1.1)BEGININSERT INTO Employee_Big_ChangeVALUES (:NEW.actionDate,:OLD.sal,:NEW.sal);

END employee_BUR;

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Triggers: Correlation Identifiers

CREATE OR REPLACE TRIGGER employee_BURBEFORE UPDATE OF SALARY ON EMPLOYEEFOR EACH ROW

DECLAREv_Sal_Difference NUMBER;

BEGINIF :old.salary <> :new.salary THEN-- do something hereEND IF;

END employee_BUR;/

Approach 1 Approach 2CREATE OR REPLACE TRIGGER

employee_BURBEFORE UPDATE OF SALARY ON EMPLOYEEFOR EACH ROWWHEN (OLD.SALARY <> NEW.SALARY)

DECLAREv_Sal_Difference NUMBER;

BEGIN-- do something here

END employee_BUR;/NOTE: When using Approach 2, do NOT

include the colons before the words OLD and NEW in the WHEN clause.

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Restrictions on Triggers Cannot have any transaction control

statements (e.g. COMMIT, ROLLBACK or SAVEPOINT)

Cannot call procedures or functions that issue any transaction control statements

Cannot declare any LONG or LONG RAW variables

:new and :old cannot refer to LONG or LONG RAW column

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Trigger Usage: Summary data: use a statement-level trigger

CREATE OR REPLACE TRIGGER patient_AIDUSAFTER INSERT OR DELETE OR UPDATE ON patient

DECLARECURSOR c_stat ISSELECT doctor_ID, COUNT(*) total_patientsFROM patientGROUP BY doctor_ID;

BEGINFOR v_StatRec IN c_stat LOOPUPDATE doctor_statsSET total_patients = v_StatRec.total_patientsWHERE Doctor_ID = v_StatRec.Doctor_ID;

IF SQL%NOTFOUND THEN INSERT INTO doctor_stats VALUES(v_StatRec.Doctor_ID, v_StatRec.total_patients);END IF;END LOOP;

END patient_AIDUS;/Another example would be in an order processing system….define the trigger on the orderDetail

table to update the total on the OrderHeader table.

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Trigger Usage: Overriding the values supplied in an INSERT/UPDATE statement by changing the :new correlation value

Use BEFORE INSERT or BEFORE UPDATE row-level triggerCREATE OR REPLACE TRIGGER patient_BIRBEFORE INSERT ON PATIENTFOR EACH ROWBEGINSELECT sequence_patientID.NEXTVALINTO :new.patientIDFROM DUAL;END patient_BIR;/

INSERT INTO patient (Fname, lname) VALUES (‘Bob’, ‘Smith’);CREATE SEQUENCE SEQUENCE_PATIENTID INCREMENT BY 1 START WITH 1;/* Note that Patient ID is automatically generated by the trigger. */

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Trigger Usage Auditing: Use AFTER Row-level

trigger

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Auditing ExampleCREATE OR REPLACE TRIGGER CALC_DOCTOR_STATS_AUIDS

AFTER INSERT OR UPDATE OR DELETE ON PATIENTDECLARE v_Dname VARCHAR2(30); v_Tot_Patients NUMBER; CURSOR C_DOCTOR_STATS IS SELECT PHY_LNAME, COUNT(SSN)

FROM PATIENT, PHYSICIAN WHERE PHY_ID = PRIMARY_PHY_ID

GROUP BY PHY_LNAME;BEGIN DELETE FROM DOCTOR_STATS; -- wipe doctor_stats records OPEN c_DOCTOR_STATS;

LOOP -- walk through cursor FETCH c_DOCTOR_STATS INTO v_Dname, v_tot_patients; EXIT WHEN c_DOCTOR_STATS%NOTFOUND;

-- create new doctor stat record for each doctor INSERT INTO DOCTOR_STATS VALUES(v_Dname, v_Tot_Patients);

END LOOP;END CALC_DOCTOR_STATS_AUIDS;

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Using Raise Application Error in TriggersCREATE OR REPLACE TRIGGER patient_bdsBEFORE DELETE ON patientDECLARE

e_weekend_error EXCEPTION;e_not_supervisor EXCEPTION;

BEGINIF TO_CHAR(SYSDATE, 'DY') = 'SAT' ORTO_CHAR(SYSDATE, 'DY') = 'SUN' THENRAISE e_weekend_error;END IF;IF SUBSTR(user, 1, 3) <> 'SUP' THENRAISE e_not_supervisor;

END IF;EXCEPTION

WHEN e_weekend_error THENRAISE_APPLICATION_ERROR (-20001, 'Deletions allowed Mon thru Fri only');WHEN e_not_supervisor THEN

-- INSERT INTO SECURITY_VIOLATIONS VALUES(USER,SYSDATE);RAISE_APPLICATION_ERROR (-20002, 'You '||user||' are not authorised to perform deletions');

END;/

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Exercise Suppose we have a Worker table as

follows:worker(workerID, lname, gender, salary, commission, deptID)

(1) Declare a sequence for workerID that begins from 1.

(2) Write a trigger that automatically inserts the primary key with a sequential number when inserting a record in the worker table.

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Exercise Trigger: Suppose we have the following two tables:

OrderHeader(OrderID, Odate, CustID, Total)Order_Item(OrderID,ItemID, Qty, Subtotal)

Write a statement-level trigger that updates the Total in the orderHeader table with the total value of the order_item records whenever an insert, update or delete event occurs on the order_item table. For any update error, raise an exception.

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Enabling & Disabling Triggers ALTER TRIGGER trigger_name

{ENABLE | DISABLE}Ex: ALTER TRIGGER Employee_BIUR DISABLE;

ALTER TABLE table_name{ENABLE | DISABLE} ALL TRIGGERS;Ex: ALTER TABLE emp ENABLE ALL TRIGGERS;

DROP TRIGGER trigger_name;Ex: DROP TRIGGER Employee_BIUR;

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Example- In order to disable, substitute DISABLE for ENABLE- By default, all triggers are enabled when they are created- When a trigger is disabled, it still exists in the DD but is never fired

- The STATUS column of USER_TRIGGERS contain either DISABLED or ENABLED value

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Dropping Triggers Dropping Triggers

DROP TRIGGER emp_bur; Trigger Dependencies

- When a table is dropped, all triggers for that table are deleted from the DD ( When a table is dropped, all packages are disabled)

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Data Dictionary user_triggers view

trigger_type, table_name, triggering_event, status To list all of the triggers that you have created:

SELECT trigger_type, table_name, triggering_event, statusFROM user_triggers;

Other views that list triggers all_triggers list triggers that are accessible to

current user (but might be owned by a different user) dba_triggers list all triggers in the database

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USER_TRIGGERSSQL> select trigger_name, trigger_type, status from user_triggers; TRIGGER_NAME TRIGGER_TYPE STATUS------------------------------ ---------------- --------LOGEMPCHANGE AFTER EACH ROW ENABLED

SQL> select trigger_name, table_name from user_triggers; TRIGGER_NAME TABLE_NAME------------------------------ ------------------------------LOGEMPCHANGE EMPLOYEE SQL> select description from user_triggers; DESCRIPTION--------------------------------------------------------------------------------LogEmpChange AFTER INSERT OR DELETE OR UPDATE ON Employee FOR EACH ROW

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Mutating Tables A table is mutating when

It is being modified by a DML statement (the table on which the trigger is defined)

It is being updated to enforce DELETE CASCADE constraints

It is being read to enforce referential integrity (RI) constraints

Constraining table A table that may need to be read from for

RI

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Mutating Tables SQL statements in Row-level trigger may NOT

Read from or modify any mutating table Read from or modify the PK, UNIQUE or FK of a

constraining table. They can, however, modify other columns in the constraining table(s).

Restrictions above apply to: all Row-level triggers Statement-level triggers ONLY when the trigger

would be fired as a result of DELETE CASCADE Single row INSERT statements do not have

this mutating table problem