DBMS中断言和触发器之间的区别

1.什么是断言?
当约束包含2个(或)更多表时，表约束机制有时会很困难，并且结果可能不会达到预期。为了解决这种情况，SQL支持创建断言，这些断言不是仅与一个表相关联的约束。断言语句应确保数据库中始终存在特定条件。只要在相应表中进行了修改，DBMS就会始终检查断言。

句法 –

CREATE ASSERTION  [ assertion_name ]
CHECK ( [ condition ] );

例子 –

CREATE TABLE sailors (sid int,sname varchar(20), rating int,primary key(sid),
CHECK(rating >= 1 AND rating <=10)
CHECK((select count(s.sid) from sailors s) + (select count(b.bid)from boats b)<100) ); 

在上面的示例中，我们强制执行CHECK约束，即船只和水手的数量应小于100。因此，在这里，我们能够同时检查两个平板电脑的约束。

2.什么是触发器?
触发器是与表相关联的数据库对象，当为表执行定义的动作时，它将被激活。当我们运行以下语句时，可以执行该触发器：

1. 插
2. 更新
3. 删除

并且可以在事件之前或之后调用它。

句法 –

create trigger [trigger_name]       
[before | after]          
{insert | update | delete} 
on [table_name]  
[for each row]    
[trigger_body]  

例子 –

create trigger t1  before  UPDATE on sailors
for each row
begin
   if new.age>60 then
      set new.age=old.age;
   else
      set new.age=new.age;
   end if;
end;
$ 

在上面的示例中，我们在更新之前创建触发器。因此，如果新年龄大于60，则不应更新，而应更新。我们可以使用“ $”符号来调用此触发器。

断言和触发器之间的区别：

S.No	Assertions	Triggers
1.	We can use Assertions when we know that the given particular condition is always true.	We can use Triggers even particular condition may or may not be true.
2.	When the SQL condition is not met then there are chances to an entire table or even Database to get locked up.	Triggers can catch errors if the condition of the query is not true.
3.	Assertions are not linked to specific table or event. It performs task specified or defined by the user.	It helps in maintaining the integrity constraints in the database tables, especially when the primary key and foreign key constraint are not defined.
4.	Assertions do not maintain any track of changes made in table.	Triggers maintain track of all changes occurred in table.
5.	Assertions have small syntax compared to Triggers.	They have large Syntax to indicate each and every specific of the created trigger.
6.	Modern databases do not use Assertions.	Triggers are very well used in modern databses.

断言不能修改数据，并且不链接到数据库中的任何特定表或事件，但是触发器具有更强大的功能，因为它们可以检查条件，还可以修改数据库内部表中的数据，这与断言不同。