如何创建触发器以检查更新的字段值

Posted 2021-04-20

创建名为products_before_update的触发器，该触发器检查Products表的discount_percent列的新值。如果折扣百分比大于100或小于0，则此触发器应引发适当的错误。如果新折扣百分比介于0和1之间，则此触发器应通过将其折叠为100来修改新折扣百分比。这样，折扣.2的百分比变为20.使用适当的UPDATE语句测试此触发器。

if语句不起作用或者我收到表正在变异的消息，因此触发器无法看到它。

connect mgs/mgs;
CREATE or replace TRIGGER products_before_update
    BEFORE UPDATE ON Products
    FOR EACH ROW IS
BEGIN
    IF :NEW.discount_percent > 100 THEN
        RAISE_APPLICATION_ERROR(-20001, 'the discount percent cannot be greater than 100.');
    ELSEIF :new.discount_percent < 0 THEN
        RAISE_APPLICATION_ERROR(-20001, 'the discount percent cannot be less than 0.');
    ELSEIF :NEW.discount_percent < 1 THEN
        SET :NEW.discount_percent = (:NEW.discount_percent * 100);
    END IF;
END;
/


SET SERVEROUTPUT ON;

UPDATE Products
SET discount_percent = .4
WHERE product_id = 3;

我期待一条消息，然后一个值超出[0,100]或更新值，当它在（0; 1）时，但触发器在任何情况下都没有反应。

答案

这是一个有效的例子。看一看。

测试用例首先：

SQL> create table products (product_id number, discount_percent number);

Table created.

SQL> insert into products values (12345, null);

1 row created.

SQL> create or replace trigger trg_prod_bu
  2    before update on products
  3    for each row
  4  begin
  5    if :new.discount_percent > 100 then
  6       raise_application_error(-20001, 'can not be greater than 100');
  7    elsif :new.discount_percent < 0 then
  8       raise_application_error(-20002, 'can not be less than 0');
  9    elsif :new.discount_percent < 1 then
 10       :new.discount_percent := :new.discount_percent * 100;
 11    end if;
 12  end;
 13  /

Trigger created.

SQL>

测试：

SQL> update products set discount_percent = -2;
update products set discount_percent = -2
       *
ERROR at line 1:
ORA-20002: can not be less than 0
ORA-06512: at "SCOTT.TRG_PROD_BU", line 5
ORA-04088: error during execution of trigger 'SCOTT.TRG_PROD_BU'


SQL> update products set discount_percent = 120;
update products set discount_percent = 120
       *
ERROR at line 1:
ORA-20001: can not be greater than 100
ORA-06512: at "SCOTT.TRG_PROD_BU", line 3
ORA-04088: error during execution of trigger 'SCOTT.TRG_PROD_BU'


SQL> update products set discount_percent = 15;

1 row updated.

SQL> update products set discount_percent = 0.2;

1 row updated.

SQL> select * From products;

PRODUCT_ID DISCOUNT_PERCENT
---------- ----------------
     12345               20

SQL>