SQL Server 2000 Trigger Enhancements

Introduction

SQL Server 2000 greatly enhances trigger functionality,
extending the capabilities of the triggers you already know and love, and adding
a whole new type of trigger, the “Instead Of” trigger.

If you are already familiar with programming triggers then
read on, but if you are new to the subject then it would be worth reading about
triggers in Books Online, or taking a look at the Triggers section on my own SQL Server page, before reading this article.

After Triggers

After Triggers are the type of trigger that existed prior to
SQL Server 2000, the “After” name is new and helps differentiate
between this type of trigger and the new “Instead Of” trigger type,
which will be covered later.

Multiple After Triggers

More than one trigger can now be defined on a table for each
Insert/Update/Delete. Although in general you might not want to do this (it’s
easy to get confused if you over-use triggers) there are situations where this
is ideal. One example that springs to mind is that you can split your triggers
up into two categories:

  • Application based triggers (cascading deletes or validation
    for example)
  • Auditing triggers (for recording details of changes to
    critical data)

This would allow you to alter triggers of one type without
fear of accidentally breaking the other.

If you are using multiple triggers, it is of course essential
to know which order they fire in. A new stored procedure called
sp_settriggerorder allows you to set a trigger to be either the
“first” or “last” to fire.

If you want more than two triggers to fire in a specific
order, there is no way to specifically define this. A deeply unscientific test I
did indicated that multiple triggers for the same table and operation will run
in the order they were created unless you specifically tell them otherwise. I
would not recommend relying on this though.

Instead Of Triggers

Instead Of Triggers fire instead of the operation that fires
the trigger, so if you define an Instead Of trigger on a table for the Delete
operation, they try to delete rows, they will not actually get deleted (unless
you issue another delete instruction from within the trigger) as in
this simple example:

create table test1 (
    vc  varchar(32)
)
go

create trigger tr_test1_o on test1 instead of delete
as
    print 'Sorry - you cannot delete this data'
go

insert test1
    select 'Cannot' union
    select 'delete' union
    select 'me'
go

delete test1
go

select * from test1
go

drop table test1

If you were to print out the contents of the Inserted and
Deleted tables from inside an Instead Of trigger you would see they behave in
exactly the same way as normal. In this case the Deleted table holds the rows
you were trying to delete, even though they will not get deleted.

Instead of Triggers can be used in some very powerful ways!

  • You can define an Instead Of trigger on a view (something
    that will not work with After triggers) and this is the basis of the
    Distributed Partitioned Views that are used so split data across a cluster
    of SQL Servers.
  • You can use Instead Of triggers to simplify the process of
    updating multiple tables for application developers.

Mixing Trigger Types

If you were to define an Instead Of trigger and an After
trigger on the same table for the same operation, what would happen?

Because an After trigger fires after an operation
completes, and an ‘instead of’ trigger prevents the operation from taking
place, the After trigger would never fire in this situation.

However, if an Instead Of trigger on a (say) delete operation
contains a subsequent delete on the same table, then any After trigger defined
for the delete operation on that table will fire on the basis of the delete
statement issued from the Instead Of trigger. The original delete statement is
not executed, only the Delete in the Instead Of trigger runs.

This code sample creates a trigger of each type, and changed
the nature of the delete statement issued so that only comics that have a value
of 0 in the preserve column can be deleted.

create table test2 (
    comic        varchar(32),
    preserve     int
)
go

insert test2
    select 'Groucho', 1 union
    select 'Chico', 1 union
    select 'Harpo', 0 union
    select 'Zeppo', 0
go

create trigger tr_test2_delete on test2 for delete
as
    select comic as "Deleting_these_names_only"
    from deleted
go

create trigger tr_test2_instead_of on test2 instead of delete
as
    delete test2
    from test2
    inner join deleted
    on test2.comic = deleted.comic
    where test2.preserve = 0
go

delete test2 where comic in ('Groucho', 'Harpo')
go

select * from test2

drop table test2

Further Reading

For more details on Instead Of triggers take a look at this article,
and of course Books Online.

Neil Boyle
Neil Boyle
Neil Boyle left school at the age of sixteen thinking that computers were things that only existed in Star Trek. After failed careers as a Diesel Mechanic, Industrial Cleaner, Barman and Bulldozer Driver he went back to college to complete his education. Since graduating from North Staffs Poly he has worked up through the ranks from Trainee COBOL Programmer to SQL Server Consultant, a role in which he has specialised for the past seven years.

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