无法理解'：new'的工作方式

if (false = FALSE) then  

    :NEW.last_modified := sysdate;
:new.ssn := null;

我们通常在行级触发器中使用:OLD引用旧值，并使用:NEW引用新值。

:OLD和:NEW的值在数据操作语言语句中可能会有所不同。

将数据插入表中时

 :OLD = NULL (since no old record)  
 :NEW = Newly inserted value

更新表中的数据时

 :OLD = Value exists in the table before the UPDATE statement executes 
 :NEW = Newly received value to replace the existing value of the record

从表中删除数据时

 :OLD = Value exists in the table before the DELETE statement executes 
 :NEW = NULL

例如，假设您要以 Alex ;

的形式将新记录插入表 Employee 中。

 :OLD = NULL (since no old record)  
 :NEW = Alex

现在，您要将值 Alex 更新为 John

 :OLD = Alex
 :NEW = John

删除此记录时，

 :OLD = John
 :NEW = NULL (since the record has been deleted)