Sql Server trigger triggers with empty inserted and deleted tables - sql-server

Question

I have defined a trigger on a table that is triggered
AFTER INSERT, DELETE, UPDATE
There are cases where the trigger fires, with both INSERTED AND DELETED tables being empty. How can this be possible?
For the records, that's the trigger
CREATE TRIGGER [dbo].[AuditUsersTrigger] ON [dbo].[Users]
AFTER INSERT, DELETE, UPDATE
AS
BEGIN
SET NOCOUNT ON
DECLARE #type nchar(1), #hasChanges bit
SET #hasChanges = 1
IF EXISTS (SELECT * FROM INSERTED)
IF EXISTS (SELECT * FROM DELETED)
BEGIN
SELECT #type = 'U'
IF EXISTS (
SELECT *
FROM INSERTED i
INNER JOIN DELETED d ON
i.Name = d.Name AND
i.Pwd = d.Pwd AND
...
) SELECT #hasChanges = 0
END
ELSE
SELECT #type = 'I'
ELSE
SELECT #type = 'D'
IF #type = 'D' OR (#type = 'U' AND #hasChanges = 1)
BEGIN
INSERT AuditUsers (
New, Id, Name, ...
)
SELECT
0, Id, Name, ...
FROM DELETED
IF #type = 'D'
BEGIN
INSERT AuditUsers (New)
SELECT 1
END
END
IF #type = 'I' OR (#type = 'U' AND #hasChanges = 1)
BEGIN
IF #type = 'I'
BEGIN
INSERT AuditUsers (New)
SELECT 0
END
INSERT AuditUsers (
New, Id, Name, ...
)
SELECT
0, Id, Name, ...
FROM INSERTED
END
IF Trigger_Nestlevel() < 2
BEGIN
DECLARE #clientId TABLE (id INT)
DECLARE #clientCode NVARCHAR(50), #shopId INT;
IF #type = 'I' OR #type = 'U'
BEGIN
SELECT #clientCode = ClientCode, #shopId = ShopId FROM INSERTED;
INSERT INTO #clientId SELECT id FROM Clients WHERE code = #clientCode;
IF NOT EXISTS (SELECT 1 FROM #clientId)
BEGIN
INSERT Clients (name, code, active, shopId) OUTPUT INSERTED.id INTO #clientId
VALUES (#clientCode, #clientCode, 1, #shopId);
END
UPDATE Users SET ClientId = (SELECT TOP 1 id FROM #clientId) WHERE ClientCode = #clientCode;
END
END
END

Answer 1

This is documented behaviour
DML triggers execute when a user tries to modify data through a data manipulation language (DML) event. DML events are INSERT, UPDATE, or DELETE statements on a table or view. These triggers fire when any valid event is fired, regardless of whether or not any table rows are affected.
If you have a recurring loop, whereby table A has a trigger that affects table B, and table B has a trigger that affects table A, you can manage this using TRIGGER_NESTLEVEL, or by checking if either inserted or deleted contain any rows before actually doing anything.