快速掌握怎样用隐性和显式空值激活触发器牛

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，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触发器激活

西庙岗乡的经验对破解“三农”问题 下列示例表明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)GOCREATE TRIGGER t1trigON t1FOR INSERT, UPDATEASIF UPDATE(a) AND UPDATE(b) PRINT \"FIRING\"GO--When two values are inserted, the UPDATE is TRUE for both columns and the trigger is SERT t1 (a, b) VALUES (1, 2) --When two values are updated, the UPDATE is TRUE for both columns and the trigger is DATE 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 SERT t1VALUES (NULL, 2)--When an explicit NULL is updated in column a, the UPDATE is TRUE for both columns,the trigger is DATE 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 SERT 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 DATE 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 SERT 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 DATE 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 SERT 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 DATE t1 SET a = 2, b = DEFAULT