Newbie on Triggers

System Sql server 2000

Hi all

I've dipped my toe in stored procedures, I'm now having a look at triggers

If I had 2 tables and I wanted to update table 2 with some data from table
1, every time table 1 has a new entry made in it (e.g. for an audit trail) I
thought I would be able to use a trigger on table 1.

for instance

on insert of new value in table 1 execute a trigger

The trigger gets the value of the newly created primary key in table 1 (the
primary key is generated by Autonumber) and inserts a copy of the primary
key into table 2

I thought the following would work

ALTER TRIGGER update_table2
ON dbo.table1
FOR INSERT AS
begin
declare @.new_key as bigint
set @.new_key = dbo.table1(PK_test_primary_key)

INSERT INTO dbo.table2
(FK_from_table1)
VALUES (@.new_key)
end

unfortunatly I cant read the value of the newly entered primary key from
table 1. I get a message saying PK_test_primary_key doesn't exist in
dbo.table1

does anyone know if what I am trying to achieve is possible, and maybe a
clue as a good way to go about doing it?

many thanks

AndyHi just a bit more...

after some more googling I have found @.@.identity and it seems to give the
identity of the last added record in the table calling the trigger

is this correct, and is it safe to do this i.e.

ALTER TRIGGER update_table2
ON dbo.table1
FOR INSERT AS
begin

INSERT INTO dbo.table2
(FK_from_table1)
VALUES (@.@.identity)
end

I've also been playing with objConn.BeginTrans in my vb6 code and I write
lots of records to table 1 before confirming the transaction. I believe that
the autonumbers are only generated on completion of the transaction, so how
many triggers will be executed (one per transaction or 1 per new record) and
at what time (as the records are written or on compltion of the
transaction)... Will it reliably update table 2 with the correct info from
table 1 if say dozens of additions are made to table 1 from different
sources at the same time...?

I think I worry too much and it seems to work when I test it, but I like to
know what happens when 8-)

thanks again

Andy

"aaj" <a.b@.c.com> wrote in message
news:4028f77b$0$29799$afc38c87@.news.easynet.co.uk. ..
> System Sql server 2000
> Hi all
> I've dipped my toe in stored procedures, I'm now having a look at triggers
> If I had 2 tables and I wanted to update table 2 with some data from table
> 1, every time table 1 has a new entry made in it (e.g. for an audit trail)
I
> thought I would be able to use a trigger on table 1.
> for instance
> on insert of new value in table 1 execute a trigger
> The trigger gets the value of the newly created primary key in table 1
(the
> primary key is generated by Autonumber) and inserts a copy of the primary
> key into table 2
> I thought the following would work
> ALTER TRIGGER update_table2
> ON dbo.table1
> FOR INSERT AS
> begin
> declare @.new_key as bigint
> set @.new_key = dbo.table1(PK_test_primary_key)
> INSERT INTO dbo.table2
> (FK_from_table1)
> VALUES (@.new_key)
> end
> unfortunatly I cant read the value of the newly entered primary key from
> table 1. I get a message saying PK_test_primary_key doesn't exist in
> dbo.table1
> does anyone know if what I am trying to achieve is possible, and maybe a
> clue as a good way to go about doing it?
> many thanks
> Andy|||aaj (a.b@.c.com) writes:
> after some more googling I have found @.@.identity and it seems to give the
> identity of the last added record in the table calling the trigger
> is this correct, and is it safe to do this i.e.
> ALTER TRIGGER update_table2
> ON dbo.table1
> FOR INSERT AS
> begin
> INSERT INTO dbo.table2
> (FK_from_table1)
> VALUES (@.@.identity)
> end

No, this is not the way to go. In a trigger you have access to two
virtual tables, inserted and deleted. inserted holds the rows that
were inserted, and in case of an UPDATE statment, the values after
the update. And deleted holds the values that were removed by a DELETE
or an UPDATE statement.

Beware that trigger fires once by statement, so many rows can be affected
at once.

--
Erland Sommarskog, SQL Server MVP, sommar@.algonet.se

Books Online for SQL Server SP3 at
http://www.microsoft.com/sql/techin.../2000/books.asp|||> ALTER TRIGGER update_table2
> ON dbo.table1
> FOR INSERT AS
> begin
> declare @.new_key as bigint
> set @.new_key = dbo.table1(PK_test_primary_key)
> INSERT INTO dbo.table2
> (FK_from_table1)
> VALUES (@.new_key)
> end
Triggers utilize the virtual "inserted" and "deleted" tables. Check
out this article:http://www.sqlteam.com/item.asp?ItemID=3850.|||You should also not that the @.@.identity is not just the last identity
created for that table, it is the last identity created. That makes the
value of @.@.identity sort of unpredictable. The other guys are right that in
the trigger you should use INSERTED and DELETED tables, but if you need to
access the latest identity created within a given stored procedure you can
use SCOPE_IDENTITY().

"aaj" <a.b@.c.com> wrote in message
news:4028fa81$0$29827$afc38c87@.news.easynet.co.uk. ..
> Hi just a bit more...
> after some more googling I have found @.@.identity and it seems to give the
> identity of the last added record in the table calling the trigger
> is this correct, and is it safe to do this i.e.
> ALTER TRIGGER update_table2
> ON dbo.table1
> FOR INSERT AS
> begin
> INSERT INTO dbo.table2
> (FK_from_table1)
> VALUES (@.@.identity)
> end
>
> I've also been playing with objConn.BeginTrans in my vb6 code and I write
> lots of records to table 1 before confirming the transaction. I believe
that
> the autonumbers are only generated on completion of the transaction, so
how
> many triggers will be executed (one per transaction or 1 per new record)
and
> at what time (as the records are written or on compltion of the
> transaction)... Will it reliably update table 2 with the correct info
from
> table 1 if say dozens of additions are made to table 1 from different
> sources at the same time...?
> I think I worry too much and it seems to work when I test it, but I like
to
> know what happens when 8-)
> thanks again
> Andy
>
> "aaj" <a.b@.c.com> wrote in message
> news:4028f77b$0$29799$afc38c87@.news.easynet.co.uk. ..
> > System Sql server 2000
> > Hi all
> > I've dipped my toe in stored procedures, I'm now having a look at
triggers
> > If I had 2 tables and I wanted to update table 2 with some data from
table
> > 1, every time table 1 has a new entry made in it (e.g. for an audit
trail)
> I
> > thought I would be able to use a trigger on table 1.
> > for instance
> > on insert of new value in table 1 execute a trigger
> > The trigger gets the value of the newly created primary key in table 1
> (the
> > primary key is generated by Autonumber) and inserts a copy of the
primary
> > key into table 2
> > I thought the following would work
> > ALTER TRIGGER update_table2
> > ON dbo.table1
> > FOR INSERT AS
> > begin
> > declare @.new_key as bigint
> > set @.new_key = dbo.table1(PK_test_primary_key)
> > INSERT INTO dbo.table2
> > (FK_from_table1)
> > VALUES (@.new_key)
> > end
> > unfortunatly I cant read the value of the newly entered primary key from
> > table 1. I get a message saying PK_test_primary_key doesn't exist in
> > dbo.table1
> > does anyone know if what I am trying to achieve is possible, and maybe a
> > clue as a good way to go about doing it?
> > many thanks
> > Andy|||Jason Sauer (jason.sauer@.fuse.net) writes:
> You should also not that the @.@.identity is not just the last identity
> created for that table, it is the last identity created. That makes the
> value of @.@.identity sort of unpredictable. The other guys are right that
> in the trigger you should use INSERTED and DELETED tables, but if you
> need to access the latest identity created within a given stored
> procedure you can use SCOPE_IDENTITY().

In this particular case @.@.identity is the only choice. scope_identity()
will return NULL, because there have been no inserts into a table
with an identity column in the current scope, that is the trigger.

The only time @.@.identity will not return the right value, is when there
are more than one trigger on the table, and the other trigger executes
first and that trigger too inserts into a table with a identity column.
Maybe not the most likely scenario.

--
Erland Sommarskog, SQL Server MVP, sommar@.algonet.se

Books Online for SQL Server SP3 at
http://www.microsoft.com/sql/techin.../2000/books.asp