創建UPDATE觸發器，導致刪除觸發行

Question

我在Oracle Express 11g上。我試圖創建此觸發器：

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是外鍵，帶有ON DELETE SET NULL子句。

當所有三個外鍵都設置爲NULL（因爲引用的行被刪除）時，我需要調查表中的一行被刪除。

觸發編譯沒有問題，但是當條件火災，然後我得到這個錯誤：

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.

其實，我知道我編輯激發觸發器在同一個表。但是我想取消更新，或者敲定它，我不在意，然後刪除那行。

你能幫我嗎？我怎樣才能實現我想要做的？

Answer 1

你可以使用一個語句級觸發器是這樣的：

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;