Triggers

What is a trigger? A trigger defines an action that the database should take when some event occurs in the application. It is triggered by an event. –An insert, update or delete statement on a specific table or view –A create, alter or drop statement on any schema object –A database startup or instance shutdown –A user logon / off The trigger works either before or after the event, according to specification.

Triggers and exceptions Often triggers are used to check to see if an event is within the domain constraints of the system: –E.g. I can’t sell stock I don’t have. –A football team cannot play itself. –If I need to reorder stock, my reorder quantity should not be less than my reorder level. Note that there are other ways to do this, other than using a trigger, but they are not defined in the DDL (data definition language, so are not as close to the data). If a trigger finds an error, it needs to inform: –The user –The DBMS

Trigger definition Create or replace trigger Before | after {delete|insert|update [of column[,column]…]} ON table-name [referencing {old [as] [new [as] ] For each row [when (condition)]] Pl/sql_block

definitions Trigger-name –is the name of the trigger. Before |after –Before indicates that the trigger should be fired before the triggering statement. –After indicates that the trigger should be fired after the triggering statement. Delete | Insert | Update [of column] –Delete indicates that the triggering statement deletes [a] row[s]. –Insert indicates that the triggering statement inserts a row. –Update indicates that the triggering statement updates the specific column(s) mentioned in the list.

Referencing –Specifies correlation names the can be used to refer to the old and new values of the row components that are being affected by the trigger. For each row –If this is used, the trigger is fired for each row that the triggering statement affects. If it is omitted, the trigger works only once per statement. When –Specifies the trigger restriction.

Example trigger create or replace trigger insert_stock before insert on stock for each row declare price_alert exception; out_of_stock exception; begin if (:new.unit_price < :new.unitcostprice) then raise price_alert; end if; if (:new.stock_level is null or :new.stock_level < :new.reorder_level) then raise out_of_stock; end if; exception when price_alert then raise_application_error(-20002,'Below cost selling'); when out_of_stock then raise_application_error(-20001,'Need to reorder stock'); when others then rollback work; end;

Example run – below cost

Stock level low

Before or after? In the case of the previous two errors, they are more informational than prohibiting, so it would be more appropriate to put them in as ‘after’ insert than before. Let’s amend the trigger, to make it operate after the insert and give a warning. –Take out exceptions –Handle warnings within code.

‘After’ trigger create or replace trigger after_stock after insert on stock for each row begin if (:new.stock_level < :new.reorder_level) then dbms_output.put_line(:new.stock_code||' being reordered.'); insert into restock values (sysdate, :new.stock_code); end if; if (:new.unit_price < :new.unitcostprice) then dbms_output.put_line(:new.stock_code||' selling below cost - record added.'); end if; exception when others then dbms_output.put_line('Unexpected error encountered - work rolled back'); rollback work; end;

Sample output

To show triggers –select trigger_name from user_triggers; To get more info: –Select trigger_name, trigger_type, triggering_event, table_name, referencing_names, trigger_body from user_triggers where trigger_name = ' ';

Write a trigger for… Write a trigger to ensure that a stock item cannot be sold for less than €1.