Delete record from one table based on column values in another

Question

I need to create a trigger of some sort that will delete an event record from my events table when the related records in my signedUp table all have a viewed field set to 1.

   events
   ----------
   eventID
   eventName
   eventLocation    

   signedUp
   ----------
   signedID
   eventID
   empID
   viewed

The viewed field defaults to 0 but gets set to 1 for a user when they view a message pertaining to the event, once all users have viewed the message all of their flags will be set to 1 and at that point I want to delete the related record from the events table, how can I create a trigger to do this?

Answer 1

Create TRIGGER [dbo].[<tr_name>]
   ON [dbo].[signedUp]
   AFTER UPDATE
AS 
BEGIN
SET NOCOUNT ON;

if exists(select s.eventID from signedUp s 
inner join inserted i on i.eventid = s.eventid
group by s.eventID
having count(s.eventID) = SUM(s.viewed))
delete e from events e inner join inserted i on i.eventid = e.eventid
END

Answer 2

Try this:

CREATE OR REPLACE TRIGGER signedup_sau -- statement after update
AFTER UPDATE ON signedup
    DELETE FROM events
    WHERE NOT EXISTS (SELECT 1
                        FROM signedup
                       WHERE signedup.eventid = events.eventid
                         AND signedup.viewed != 1);
END;

The trigger has to run after the entire UPDATE statement runs, as it is neither possible nor logical to reference the same table in a row level trigger.