Using Nested Triggers
Triggers are nested when a trigger performs an action that initiates another trigger, which can initiate another trigger, and so on. Triggers can be nested up to 32 levels, and you can control whether triggers can be nested through the nested triggers server configuration option.
If nested triggers are allowed and a trigger in the chain starts an infinite loop, the nesting level is exceeded and the trigger terminates.
You can use nested triggers to perform useful housekeeping functions such as storing a backup copy of rows affected by a previous trigger. For example, you can create a trigger on titleauthor that saves a backup copy of the titleauthor rows that the delcascadetrig trigger deleted. With the delcascadetrig trigger in effect, deleting title_id PS2091 from titles deletes the corresponding row or rows from titleauthor. To save the data, you create a DELETE trigger on titleauthor that saves the deleted data into another separately created table, del_save. For example:
CREATE TRIGGER savedel ON titleauthor FOR DELETE AS INSERT del_save SELECT * FROM deleted
Using nested triggers in an order-dependent sequence is not recommended. Use separate triggers to cascade data modifications.
Note Because triggers execute within a transaction, a failure at any level of a set of nested triggers cancels the entire transaction, and all data modifications are rolled back. Include PRINT statements in your triggers so that you can determine where the failure occurred.
A trigger does not call itself recursively unless the RECURSIVE_TRIGGERS database option is set. There are two types of recursion:
- Direct recursion
Occurs when a trigger fires and performs an action that causes the same trigger to fire again. For example, an application updates table T3, which causes trigger Trig3 to fire. Trig3 updates table T3 again, which causes trigger Trig3 to fire again.
- Indirect recursion
Occurs when a trigger fires and performs an action that causes another trigger (either on the same table or on another table) to fire. This second trigger performs an action that causes the original trigger to fire again. For example, an application updates table T1, which causes trigger Trig1 to fire. Trig1 updates table T2, which causes trigger Trig2 to fire. Trig2 in turn updates table T1 that causes Trig1 to fire again.
Only direct recursion is prevented when the RECURSIVE_TRIGGERS database option is set to OFF. To disable indirect recursion, set the nested triggers server option to 0, as well.
A. Use recursive triggers to solve self-referencing relationships
One use for recursive triggers is on a table with a self-referencing relationship (also known as transitive closure). For example, the table emp_mgr defines:
- An employee (emp) in a company.
- The manager for each employee (mgr).
- The total number of employees in the organizational tree reporting to each employee (NoOfReports).
A recursive UPDATE trigger can be used to keep the NoOfReports column up-to-date as new employee records are inserted. The INSERT trigger updates the NoOfReports column of the manager record, which recursively updates the NoOfReports column of other records up the management hierarchy.
USE pubs GO -- Turn recursive triggers ON in the database. ALTER DATABASE pubs SET RECURSIVE_TRIGGERS ON GO CREATE TABLE emp_mgr ( emp char(30) PRIMARY KEY, mgr char(30) NULL FOREIGN KEY REFERENCES emp_mgr(emp), NoOfReports int DEFAULT 0 ) GO CREATE TRIGGER emp_mgrins ON emp_mgr FOR INSERT AS DECLARE @e char(30), @m char(30) DECLARE c1 CURSOR FOR SELECT emp_mgr.emp FROM emp_mgr, inserted WHERE emp_mgr.emp = inserted.mgr OPEN c1 FETCH NEXT FROM c1 INTO @e WHILE @@fetch_status = 0 BEGIN UPDATE emp_mgr SET emp_mgr.NoOfReports = emp_mgr.NoOfReports + 1 -- Add 1 for newly WHERE emp_mgr.emp = @e -- added employee. FETCH NEXT FROM c1 INTO @e END CLOSE c1 DEALLOCATE c1 GO -- This recursive UPDATE trigger works assuming: -- 1. Only singleton updates on emp_mgr. -- 2. No inserts in the middle of the org tree. CREATE TRIGGER emp_mgrupd ON emp_mgr FOR UPDATE AS IF UPDATE (mgr) BEGIN UPDATE emp_mgr SET emp_mgr.NoOfReports = emp_mgr.NoOfReports + 1 -- Increment mgr's FROM inserted -- (no. of reports) by WHERE emp_mgr.emp = inserted.mgr -- 1 for the new report. UPDATE emp_mgr SET emp_mgr.NoOfReports = emp_mgr.NoOfReports - 1 -- Decrement mgr's FROM deleted -- (no. of reports) by 1 WHERE emp_mgr.emp = deleted.mgr -- for the new report. END GO -- Insert some test data rows. INSERT emp_mgr(emp, mgr) VALUES ('Harry', NULL) INSERT emp_mgr(emp, mgr) VALUES ('Alice', 'Harry') INSERT emp_mgr(emp, mgr) VALUES ('Paul', 'Alice') INSERT emp_mgr(emp, mgr) VALUES ('Joe', 'Alice') INSERT emp_mgr(emp, mgr) VALUES ('Dave', 'Joe') GO SELECT * FROM emp_mgr GO -- Change Dave's manager from Joe to Harry UPDATE emp_mgr SET mgr = 'Harry' WHERE emp = 'Dave' GO SELECT * FROM emp_mgr GO
Here are the results before the update:
emp mgr NoOfReports ------------------------------ ----------------------------- ----------- Alice Harry 2 Dave Joe 0 Harry NULL 1 Joe Alice 1 Paul Alice 0
Here are the results after the update:
emp mgr NoOfReports ------------------------------ ----------------------------- ----------- Alice Harry 2 Dave Harry 0 Harry NULL 2 Joe Alice 0 Paul Alice 0
To set the nested triggers option
To set the recursive triggers database option