创建UPDATE触发器，导致删除触发行

Question

我在Oracle Express 11g上。我试图创建此触发器：

CREATE TRIGGER remove_useless_surveys 
    BEFORE UPDATE OF agent_id, agency_id, province_id ON surveys 
    FOR EACH ROW WHEN (
    new.agent_id IS NULL AND 
    new.agency_id IS NULL AND 
    new.province_id IS NULL) 
DELETE FROM surveys WHERE survey_code = :new.survey_code 

agent_id，agency_id和province_id是外键，带有ON DELETE SET NULL子句。

当所有三个外键都设置为NULL（因为引用的行被删除）时，我需要调查表中的一行被删除。

触发编译没有问题，但是当条件火灾，然后我得到这个错误：

SQL error: ORA-04091: table REALESTATE.SURVEYS is mutating,
trigger/function may not see it ORA-06512: at
"REALESTATE.REMOVE_USELESS_SURVEYS", line 1 ORA-04088: error during
execution of trigger 'REALESTATE.REMOVE_USELESS_SURVEYS'
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.

其实，我知道我编辑激发触发器在同一个表。但是我想取消更新，或者敲定它，我不在意，然后删除那行。

你能帮我吗？我怎样才能实现我想要做的？

Answer 1

你可以使用一个语句级触发器是这样的：

CREATE TRIGGER remove_useless_surveys 
AFTER UPDATE OF agent_id, agency_id, province_id ON surveys 
BEGIN 
    DELETE FROM surveys 
    WHERE agent_id IS NULL 
    AND agency_id IS NULL 
    AND province_id IS NULL; 
END;