SQL Server “AFTER INSERT” trigger doesn't see the just-inserted row

Question

Consider this trigger:

ALTER TRIGGER myTrigger 
   ON someTable 
   AFTER INSERT
AS BEGIN
  DELETE FROM someTable
         WHERE ISNUMERIC(someField) = 1
END         


        

Answer 1

I stumbled across this question looking for details on the sequence of events during an insert statement & trigger. I ended up coding some brief tests to confirm how SQL 2016 (EXPRESS) behaves - and thought it would be appropriate to share as it might help others searching for similar information.

Based on my test, it is possible to select data from the "inserted" table and use that to update the inserted data itself. And, of interest to me, the inserted data is not visible to other queries until the trigger completes at which point the final result is visible (at least best as I could test). I didn't test this for recursive triggers, etc. (I would expect the nested trigger would have full visibility of the inserted data in the table, but that's just a guess).

For example - assuming we have the table "table" with an integer field "field" and primary key field "pk" and the following code in our insert trigger:

select @value=field,@pk=pk from inserted
update table set field=@value+1 where pk=@pk
waitfor delay '00:00:15'

We insert a row with the value 1 for "field", then the row will end up with the value 2. Furthermore - if I open another window in SSMS and try: select * from table where pk = @pk

where @pk is the primary key I originally inserted, the query will be empty until the 15 seconds expire and will then show the updated value (field=2).

I was interested in what data is visible to other queries while the trigger is executing (apparently no new data). I tested with an added delete as well:

select @value=field,@pk=pk from inserted
update table set field=@value+1 where pk=@pk
delete from table where pk=@pk
waitfor delay '00:00:15'

Again, the insert took 15sec to execute. A query executing in a different session showed no new data - during or after execution of the insert + trigger (although I would expect any identity would increment even if no data appears to be inserted).