SQL Server 2008 R2 : Transactions and Triggers (part 2) - Triggers and Multistatement Transactions, Using Savepoints in Triggers

2. Triggers and Multistatement Transactions

Now let’s look at another example. First, you need to create a trigger to enforce referential integrity between the titles table and publishers table:

--The first statement is used to disable any previously created
--DDL triggers in the database which would prevent creating a new trigger.
DISABLE TRIGGER ALL ON titles
go
create trigger tr_titles_i on titles for insert as
declare @rows int  — create variable to hold @@rowcount
select @rows = @@rowcount
if @rows = 0 return
if update(pub_id) and (select count(*)
        from inserted i, publishers p
        where p.pub_id = i.pub_id ) != @rows
  begin
        rollback transaction
        raiserror ('Invalid pub_id inserted', 16, 1)
  end
return
go

Next, for the trigger to take care of the referential integrity, you might first need to disable the foreign key constraint on the titles table with a command similar to the following:

alter table titles nocheck constraint FK__titles__pub_id__0F424F67

Now, run a multistatement transaction with an invalid pub_id in the second insert statement:

/* transaction inserts rows into a table */
begin tran add_titles
insert titles (title_id, pub_id, title)
       values ('XX1234', '0736', 'Tuning SQL Server')
insert titles (title_id, pub_id, title)
       values ('XX1235', 'abcd', 'Tuning SQL Server')
insert titles (title_id, pub_id, title)
       values ('XX1236', '0877', 'Tuning SQL Server')
commit tran
go

Msg 50000, Level 16, State 1, Procedure tr_titles_i, Line 10
Invalid pub_id inserted
Msg 3609, Level 16, State 1, Line 4
The transaction ended in the trigger. The batch has been aborted.

How many rows are inserted if 'abcd' is an invalid pub_id? In this example, no rows are inserted because the ROLLBACK TRAN in the trigger rolls back all modifications made by the trigger, including the insert with the bad pub_id and all statements preceding it within the transaction. After the RETURN statement is encountered in the trigger, the rest of the batch is aborted.

3. Using Savepoints in Triggers

Although BEGIN TRAN statements are not recommended within a trigger, you can set a savepoint in a trigger and roll back to the savepoint. This technique rolls back only the operations within the trigger subsequent to the savepoint. The trigger and transaction it is a part of are still active until the transaction is subsequently committed or rolled back. The batch continues processing.

Savepoints can be used to avoid a trigger’s arbitrarily rolling back an entire transaction. You can roll back to the named savepoint in the trigger and then issue a raiserror and return immediately to pass the error code back to the calling process. The calling process can then check the error status of the data modification statement and take appropriate action, either rolling back the transaction, rolling back to a savepoint in the transaction, or ignoring the error and committing the data modification.

The following example shows a trigger that uses a savepoint:

alter trigger tr_titles_i on titles for insert as
declare @rows int  — create variable to hold @@rowcount
select @rows = @@rowcount
if @rows = 0 return
save tran titlestrig
if update(pub_id) and (select count(*)
        from inserted i, publishers p
        where p.pub_id = i.pub_id ) != @rows
  begin
       rollback transaction titlestrig
       raiserror ('Invalid pub_id inserted', 16, 1)
  end
return

This trigger rolls back all work since the savepoint and returns an error number of 50000. In the transaction, you can check for the error number and make the decision about whether to continue the transaction, roll back the transaction, or, if savepoints were set in the transaction, roll back to a savepoint and let the transaction continue. The following example rolls back the entire transaction if either of the first two inserts fail, but it rolls back to the named savepoint only if the third insert fails, allowing the first two to be committed:

begin tran add_titles
insert titles (title_id, pub_id, title)
       values ('XX1234', '0736', 'Tuning SQL Server')
if @@error = 50000 — roll back entire transaction and abort batch
   begin
   rollback tran add_titles
   return
   end
insert titles (title_id, pub_id, title)
       values ('XX1236', '0877', 'Tuning SQL Server')
 if @@error = 50000 — roll back entire transaction and abort batch
   begin
   rollback tran add_titles
   return
   end
save tran keep_first_two  — set savepoint for partial rollback
insert titles (title_id, pub_id, title)
       values ('XX1235', 'abcd', 'Tuning SQL Server')
 if @@error = 50000  — roll back to save point, continue batch
   begin
   rollback tran keep_first_two
   end
commit tran


					  

Note

Don’t forget to reenable the constraint on the titles table when you are finished testing:

alter table titles check constraint FK__titles__pub_id__0F424F67