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Asked: 2026-05-11T01:53:50+00:00

Suppose you have the tables Presentations and Events . When a presentation is saved

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Suppose you have the tables Presentations and Events. When a presentation is saved and contains basic event information, such as location and date, an event will be created automatically using a trigger. (I’m afraid it’s impossible for technical reasons to simply keep the data at one place and use a view.) In addition, when changing this information later on in the presentation, the trigger will copy the updates over to the event as well, like so:

CREATE TRIGGER update_presentations ON Presentations AFTER UPDATE AS BEGIN     UPDATE Events     SET Events.Date = Presentations.Date,         Events.Location = Presentations.Location     FROM Presentations INNER JOIN Events ON Presentations.EventID = Events.ID     WHERE Presentations.ID IN (SELECT ID FROM inserted) END

Now, the customer wants it so that, if a user ever changes the information in the event, it should go back to the presentation as well. For obvious reasons, I can’t do the reverse:

CREATE TRIGGER update_events ON Events AFTER UPDATE AS BEGIN     UPDATE Presentations     SET Presentations.Date = Events.Date,         Presentations.Location = Events.Location     FROM Events INNER JOIN Presentations ON Events.PresentationID = Presentations.ID     WHERE Events.ID IN (SELECT ID FROM inserted) END

After all, this would cause each trigger to fire after each other. What I could do is add a column last_edit_by to both tables, containing a user ID. If filled by the trigger with a special invalid ID (say, by making all user IDs of actual persons positive, but user IDs of scripts negative), I could use that as an exit condition:

    AND last_edit_by >= 0

This might work, but what I’d like to do is indicate to the SQL server that, within a transaction, a trigger should only fire once. Is there a way to check this? Or perhaps to check that a table has already been affected by a trigger?

Answer thanks to Steve Robbins:

Just wrap the potentially nested UPDATE statements in an IF condition checking for trigger_nestlevel(). For example:

CREATE TRIGGER update_presentations ON Presentations AFTER UPDATE AS BEGIN     IF trigger_nestlevel() < 2         UPDATE Events         SET Events.Date = Presentations.Date,             Events.Location = Presentations.Location         FROM Presentations INNER JOIN Events ON Presentations.EventID = Events.ID         WHERE Presentations.ID IN (SELECT ID FROM inserted) END

Note that trigger_nestlevel() appears to be 1-based, not 0-based. If you want each of the two triggers to execute once, but not more often, just check for trigger_nestlevel() < 3 in both triggers.

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1 Answer

  1. I’m not sure about doing it per transaction, but do you need nested triggers switched on for other parts? If you switch them off on the server then a trigger won’t fire from another trigger updating a table.

    EDIT (answer from the comments): You will need to alter trigger A to use TRIGGER_NESTLEVEL

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