Question

I have some problem executing the trigger below:

CREATE OR REPLACE TRIGGER AFTERINSERTCREATEBILL
AFTER INSERT
ON READING
FOR EACH ROW 

DECLARE

varReadNo   Int;
varMeterID  Int;
varCustID   Varchar(10);

BEGIN 

SELECT SeqReadNo.CurrVal INTO varReadNo FROM DUAL;

Select MeterID INTO varMeterID
From Reading 
Where ReadNo = varReadNo;

Select CustID INTO varCustID
From Address A
Join Meter M 
on A.postCode = M.postCode
Where M.MeterID = varMeterID;

INSERT INTO BILL VALUES 
(SEQBILLNO.NEXTVAL, SYSDATE, 'UNPAID' , 100 , varCustID , SEQREADNO.CURRVAL); 

END;

Error Message:

*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.

Does it mean that I am not suppose to retrieve any details from table Reading?

I believe that the issue occur as I retrieving data from Table Reading while it is inserting the value:

SELECT SeqReadNo.CurrVal INTO varReadNo FROM DUAL;

Select MeterID INTO varMeterID
From Reading 
Where ReadNo = varReadNo; 

It’s there anyway to resolve this? Or is there a better method to do this? I need to insert a new row into table BILL after entering the table Reading where the ReadNo need to reference to the ReadNo I just insert.

Answer 1

You cannot retrieve records from the same table in a row trigger. You can access values from actual record using :new and :old (is this your case?). The trigger could then be rewritten to

CREATE OR REPLACE TRIGGER AFTERINSERTCREATEBILL
AFTER INSERT
ON READING
FOR EACH ROW 

DECLARE

  varCustID   Varchar(10);

BEGIN 

  Select CustID INTO varCustID
    From Address A
    Join Meter M 
      on A.postCode = M.postCode
    Where M.MeterID = :new.MeterID;

  INSERT INTO BILL VALUES 
  (SEQBILLNO.NEXTVAL, SYSDATE, 'UNPAID' , 100 , varCustID , SEQREADNO.CURRVAL); 

END;

If you need to query other record from READING table you have to use a combination of statement triggers, row trigger and a PLSQL collection. Good example of this is on AskTom.oracle.com