1. Triggers
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2. Objective To develop an understanding of event based triggers.
To explore the role of triggers in the construction of transactions.
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3. Triggers
A trigger is a routine which is associated with a database object; typically a table.
A trigger is executed 'fired' when some condition becomes true.
So, a trigger is implicitly executed rather than called like a function or procedure.
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4. Triggers have the property of atomicity like transactions- wholly successful or not.
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5. Contains any new rows for a table. Created by insert actions.
At the point a trigger intercepts an operation two temporary tables exist.
 Inserted table
 Contains any new rows for a table. Created by insert actions.
 Deleted table
 Contains rows for removal. Created by delete actions
 Update actions create both an inserted table and deleted table. New rows in inserted, and old rows in deleted.
 Tables only exist for scope of the event and are removed when all triggers for that event are complete and data committed.
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6. So, conditions (If’s) must take this into account.
When programming a trigger the data being inserted, deleted or updated must be assumed to be in the table until transaction is complete (committed).
So, conditions (If’s) must take this into account.
E.g. if testing that data to be inserted does not conflict with other data the trigger must assume that the new data is already inserted.
So, any query conditions in the trigger will be tested against the new data as well as the existing data.
Trigger can terminate and rollback the transaction, thereby removing the inserted data if an error is detected
Transaction not committed until all triggers complete.
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7. SQL*Server Triggers A type of stored procedure.
Executed automatically when a table updated.
 Cannot pass values in and out of triggers.
Create trigger trigger_name On table_name
For {insert, delete, update}
As sql_statements;
 Triggers removed using-
drop trigger <name>
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8. Dropping a table removes all associated triggers-
this means that a trigger definition separate from the table is needed so that they can be easily re-created and re-used
(hence better to manage triggers using Query Analyser rather than Access 2000.)
Triggers are a component of schemas.
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9. Example- to reject some invalid data
@ designates a parameter.
Create trigger tri_ins_sale On sales
For insert, update As
= datepart(day, I.ord_date) 
{place the day as an integer from the column ord_date from table I in parameter}
From sales s, inserted I
Where s.stor_id=I.stor_id
And s.ord_num=I.ord_num
And s.title_id=I.title_id
Begin
{Test the day number and reject transaction if appropriate}
Rollback tran
{Rolls back to last BEGIN TRANS or default equivalent}
Raiseerror ('orders must be placed before 15th of month, 16, 10)
End Go
Detailed syntax in Transact*SQL Help under Triggers.
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10. Cascading Referential Actions using Triggers
This is only relevant to SQL*Server 7.
(Referential actions should be declared using foreign key declarations in SQL*Server 2000)
Referential Integrity
 SQL*Server 7 does not support cascade as a referential operation.
This can be dealt with using a trigger so long as no referential constraints (foreign keys) are declared in the database schema.
This is because violation of the declared constraint is detected before the trigger fires.
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11. Cascades
e.g. to cascade update on a customer table to a related order table.
Order contains the foreign key customerid (for the primary key of customer).
This cascades an update to the customer table into the order table
i.e. a change to a customerid in the customer table results in changes to the associated customerid's in the order table.
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12. create trigger update_customer on customer for update
nchar(5)
=customerid from deleted
=customerid from inserted
if  exists(select * from orders where
update orders
set
where
Syntax and method the same for cascaded deletes.
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13. Summary
SQL based routines can be executed implicitly as a result of an action or some condition becoming true (‘triggered’)
Used to protect the integrity of the database or to add value to database operations by automating complex activities
This can produce a cascade of database operations
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