Question

我有两个表，PRODUCTS和STATE_PRICE。每种产品的价格因州而异。 PRODUCTS表跟踪所有州的每种产品的平均成本。我正在尝试编写一个触发器，当在STATE_PRICE表中插入，更新或删除价格时，该触发器将更新PRODUCTS表中项目的平均价格。我编写了以下触发器，它编译，但当我测试它时，我得到一个变异的错误消息。我理解变异错误的概念，我正在尝试更新正在执行触发器的表，但我实际上是在STATE_PRICE表上执行触发器时尝试更新PRODUCTS表。

create or replace trigger trg_avg_cost
after insert or update or delete on state_price
for each row

declare
w_price state_price.list_price%type;
w_product state_price.productid%type;

begin
w_price := :new.list_price;
w_product := :new.productid;

update products
set avg_cost_per_unit = (select avg(w_price) from state_price
where productid = w_product);

end;
/

我得到的具体错误消息是：

错误报告：

SQL Error: ORA-04091: table STATE_PRICE is mutating, trigger/function may not see it
ORA-06512: at "TRG_AVG_COST", line 9
ORA-04088: error during execution of trigger 'TRG_AVG_COST'
04091. 00000 -  "table %s.%s is mutating, trigger/function may not see it"
*Cause:    A trigger (or a user defined plsql function that is referenced in
           this statement) attempted to look at (or modify) a table that was
           in the middle of being modified by the statement which fired it.
*Action:   Rewrite the trigger (or function) so it does not read that table.

Answer 1

可能存在参照完整性约束（在产品ID上），这也可能引发相同的错误。如果是这种情况，以下链接可以帮助您避免错误。

http://asktom.oracle.com/pls/asktom/ASKTOM.download_file?p_file=6551198119097816936

Answer 2

在行触发器中，没有SQL语句可以访问触发器所在的表。您的SELECT AVG(W_PRICE) FROM STATE_PRICE WHERE PRODUCTID = W_PRODUCT是造成错误的原因。解决此限制的经典方法是使用复合触发器 - 文档here。另请参阅我对this StackOverflow question的回答，了解实现复合触发器的示例。

分享并享受。

突变错误触发器计算平均成本

2 个答案: