In SQL Server, the term "nesting level" refers to the depth of nested actions triggered by triggers. When a trigger on a table causes another trigger to be fired, this creates a nesting of triggers. The function TRIGGER_NESTLEVEL() is used to determine the level of nesting for the trigger currently being executed.
Here's a bit more detail on TRIGGER_NESTLEVEL():
Purpose: TRIGGER_NESTLEVEL() is a function in SQL Server that returns the number of triggers executed in a chain of nested triggers. This helps in managing and controlling recursive or nested triggers execution.
Usage: You can use TRIGGER_NESTLEVEL() to:
Prevent excessive nesting which could lead to stack overflow, performance degradation, or logical errors in the application. Control or limit the recursive behavior of triggers by setting a maximum allowed nesting level. Example: In the trigger code you provided, the line IF TRIGGER_NESTLEVEL() > 1 RETURN; checks if the current trigger is nested inside another trigger. If the nesting level is greater than 1, it stops executing the trigger (RETURN;). This prevents recursive updates that could happen if the trigger modifies the table it's associated with, which in turn would fire the trigger again, creating a potential for infinite recursion.
In summary, the concept of nest levels in SQL Server triggers is crucial for managing the behavior of triggers, especially to avoid unwanted recursive trigger executions that could lead to performance issues or unexpected behaviors.