下文将为您详细解读隐性和显式空值激活sql server触发器的方法，供您参考，希望对您学习sql server触发器的使用能够有所启迪。
在列中插入显式空值，或使用 DEFAULT 关键字为列赋值，都可以按预期激活触发器。同样，当没有在 INSERT 语句中为列指定值时，sql server触发器仍可以在下列条件下激活：
◆由于不存在DEFAULT 定义，列中插入了一个隐性空值。
◆由于DEFAULT 定义确实存在，列中插入了一个默认值。
示例：用空值和默认值测试sql server触发器激活
下列示例表明sql server触发器如何受隐性和显式空值的影响。创建小型表以容纳两个触发器的值。一列包含空值，另一列包含默认值。触发器评价上述两列是否需要修改并且显示激活触发器时的信息。一系列 INSERT 语句通过插入隐性和显式空值的组合来测试触发器激活。
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