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Active Database Concepts
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Active Databases Active Rules – rules that are automatically triggered by events in the database.
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Event-Condition-Action
if(event && condition) { action; } Event – an event occurs that triggers a rule Condition – check to see if the rule should be executed Action – the action to be taken
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Event e.g. INSERT, UPDATE, DELETE. Types of Events Event granularity
isolated event transaction or chain reaction Event granularity Row level - tuples Statement level – statements
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Condition When do you consider the condition?
Immediate – when the event happens. Deferred – at the end of a transaction. Detached – in a separate transaction spawned by the trigger.
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Immediate Consideration
Three flavors Before – e.g. date modified After – e.g. transaction log Instead of – e.g. view Oracle uses this model.
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Deferred Consideration
Check all of the conditions at the end of a transaction. You could have transient data that you don't want triggering an event. e.g. Two students switching classes.
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SQL3 Trigger Syntax CREATE TRIGGER name
{BEFORE|AFTER} <event> ON table [REFERENCING <alias list> ] [FOR EACH [ROW|STATEMENT]] [WHEN (condition)] <body>
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Oracle Syntax CREATE [OR REPLACE] TRIGGER name
{BEFORE|AFTER|INSTEADOF} {DELETE|INSERT|UPDATE[OF column_list]} ON table_name [ REFERENCING [ OLD AS old_var ] [ NEW AS new_var ] ] [FOR EACH ROW [ WHEN (condition)] trigger PL/SQL body;
![Oracle Syntax CREATE [OR REPLACE] TRIGGER name](http://slideplayer.com./slide/13278617/79/images/9/Oracle+Syntax+CREATE+%5BOR+REPLACE%5D+TRIGGER+name.jpg "{BEFORE|AFTER|INSTEADOF} {DELETE|INSERT|UPDATE[OF column_list]} ON table_name. [ REFERENCING [ OLD AS old_var ] [ NEW AS new_var ] ] [FOR EACH ROW [ WHEN (condition)] trigger PL/SQL body;")
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{BEFORE|AFTER|INSTEADOF}
Timing Options {BEFORE|AFTER|INSTEADOF} BEFORE – before the triggering event makes any changes to the database. You can alter the triggering event. AFTER – executes after the triggering event is processed. Can't alter the triggering event INSTEAD OF – do something other than the triggering event. Map an insertion on a view to physical tables.
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Triggering Statement {DELETE|INSERT|UPDATE[OF column_list]} ON table_name The type of SQL statement that fires the trigger body. The name of the table UPDATE can limit the firing scope to just columns.
![Triggering Statement {DELETE|INSERT|UPDATE[OF column_list]} ON table_name. The type of SQL statement that fires the trigger body.](http://slideplayer.com./slide/13278617/79/images/11/Triggering+Statement+%7BDELETE%7CINSERT%7CUPDATE%5BOF+column_list%5D%7D+ON+table_name.+The+type+of+SQL+statement+that+fires+the+trigger+body..jpg "The name of the table. UPDATE can limit the firing scope to just columns.")
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FOR EACH ROW Option [FOR EACH ROW [ WHEN (condition)] FOR EACH ROW - determines if you are using a statement or row level trigger WHEN clause– a boolean condition to further restrict the trigger. You can't use methods or stored procedures.
![FOR EACH ROW Option [FOR EACH ROW [ WHEN (condition)] FOR EACH ROW - determines if you are using a statement or row level trigger.](http://slideplayer.com./slide/13278617/79/images/12/FOR+EACH+ROW+Option+%5BFOR+EACH+ROW+%5B+WHEN+%28condition%29%5D+FOR+EACH+ROW+-+determines+if+you+are+using+a+statement+or+row+level+trigger..jpg "WHEN clause– a boolean condition to further restrict the trigger. You can t use methods or stored procedures.")
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REFERENCING Option [ REFERENCING [ OLD AS old_var ] [ NEW AS new_var ] ] If you have a row level trigger you can use :old and :new, or your alias, to reference the pre-change and post-change values respectively You can only do this with INSERT, UPDATE, and DELETE tuples :old for INSERT? :new for DELETE?
![REFERENCING Option [ REFERENCING [ OLD AS old_var ] [ NEW AS new_var ] ]](http://slideplayer.com./slide/13278617/79/images/13/REFERENCING+Option+%5B+REFERENCING+%5B+OLD+AS+old_var+%5D+%5B+NEW+AS+new_var+%5D+%5D.jpg "If you have a row level trigger you can use :old and :new, or your alias, to reference the pre-change and post-change values respectively. You can only do this with INSERT, UPDATE, and DELETE tuples. :old for INSERT :new for DELETE")
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Trigger Body Trigger bodies can contain DML SQL statements (INSERT, DELETE, UPDATE) SELECT INTO or SELECT w/ cursors No DDL allowed (CREATE, DROP, ALTER) Conditional Predicate IF INSERTING THEN … END IF; IF UPDATING ('EID') THEN … END IF; IF DELETING THEN … END IF;
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Mutating tables Mutating table = table that is currently being modified by an INSERT, UPDATE, or DELETE You can't look at a table as it is mutating because you can get inconsistent data. Statement-triggers don't have this problem as long as they aren't fired from a DELETE CASCADE.
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Constraining Tables Constraining table is a table that a triggering statement reads using SQL or referential integrity. Triggers can't change PRIMARY, FOREIGN, OR UNIQUE KEY columns of a constraining table. One exception – BEFORE ROW and AFTER ROW single row INSERT statements.
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Trigger Firing Order BEFORE statement trigger For each row
BEFORE row trigger Triggering statement AFTER row trigger AFTER statement trigger
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Consistency and Termination
Trigger failure results in a data rollback. Triggers can create a chain reaction of cascading triggers. Cascading triggers can create loops. CREATE OR REPLACE TRIGGER loop_ais AFTER INSERT ON loop BEGIN INSERT INTO loop values(1); END;
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