快速掌握怎样用隐性和显式空值激活触发器

CREATE TABLE t1
(a int NULL, b int NOT NULL DEFAULT 99)
GO

CREATE TRIGGER t1trig
ON t1
FOR INSERT, UPDATE
AS
IF UPDATE(a) AND UPDATE(b)
   PRINT 'FIRING'
GO

--When two values are inserted, the UPDATE is TRUE for 
both columns and the trigger is activated.
INSERT t1 (a, b) 
VALUES (1, 2) 

--When two values are updated, the UPDATE is TRUE for 
both columns and the trigger is activated.
UPDATE t1 
SET a = 1, b = 2

--When an explicit NULL is inserted in column a,
the UPDATE is TRUE for both columns and the trigger is activated.
INSERT t1
VALUES (NULL, 2)

--When an explicit NULL is updated in column a, 
the UPDATE is TRUE for both columns,the trigger is activated.
UPDATE t1 
SET a = NULL, b = 2

--When an implicit NULL is inserted in column a,
 the UPDATE is TRUE for both columns and the trigger is activated.
INSERT t1 (b)
VALUES (2)

--When column a is updated with an implicit NULL,
 the UPDATE is FALSE for both columns and the trigger is not activated.
UPDATE t1 
SET b = 2

--When the default value is implicitly inserted in column b, 
the UPDATE is TRUE for both columns and the trigger is activated.
INSERT t1 (a)
VALUES (2)

--When column b is updated with an implicit NULL, 
the UPDATE is FALSE for both columns and the trigger is not activated.
UPDATE t1 
SET a = 2

--When the default value is explicitly inserted in column b, 
the UPDATE is TRUE for both columns and the trigger is activated.
INSERT t1 (a, b)
VALUES (2, DEFAULT)

--When column b is updated explicitly with the default value, 
the UPDATE is TRUE for both columns and the trigger is activated.
UPDATE t1 
SET a = 2, b = DEFAULT