How to avoid "table mutating" errors - sql

Question

I have an trigger that needs to read from a table after deleting a row. Essentially, I need to count up the remaining rows that are similar to the current row, and if that count is zero, update a field elsewhere.
After two days of hammering around, I haven't been able to figure out how to restructure my thought process to allow me to do this. Here is an example:
CREATE OR REPLACE TRIGGER Di_PatMustBeWell
AFTER DELETE
ON Diagnosis
FOR EACH ROW
Declare
--PRAGMA AUTONOMOUS_TRANSACTION;
NumDiseases Number;
BEGIN
SELECT NUMDISEASES INTO Numdiseases
FROM DiagnosisCount
where Di_Patient = :OLD.Di_Patient;
IF( NumDiseases != 1 ) THEN
UPDATE Patient SET Pat_Sick = 0 WHERE Pat_Person = :OLD.Di_Patient;
END IF;
END;
/

Answer 1

Short answer - no trigger, no mutating.
Yow can use the trigger with pragma autonomous_transaction for counting of remaining diagnoses for certain patient, but it's is not recommended way to do this.
Better you create new function or procedure to implement your logic on deleted diagnosis. Something like this:
create table Diagnosis as select 456 idDiseases, 123 di_patient from dual;
/
create table diagnosisCount as select 1 numDiseases, 123 di_patient from dual;
/
create table Patient as select 123 Pat_Person, 1 Pat_Sick from dual;
/
drop trigger di_patmustbewell;
create or replace function deleteDiagnosis(idDiseases number) return number is
rows_ number;
di_patient number;
Numdiseases number;
begin
<<del>> begin
delete Diagnosis where IdDiseases = deleteDiagnosis.IdDiseases
returning Diagnosis.di_patient into deleteDiagnosis.di_patient
;
rows_ := sql%rowcount;
if rows_ != 1 then raise too_many_rows; end if;
end del;
select count(1) into deleteDiagnosis.numDiseases from Diagnosis where Di_Patient = deleteDiagnosis.di_patient;
if deleteDiagnosis.numdiseases = 0 then <<upd>> begin
update Patient set Pat_Sick = 0 where Pat_Person = deleteDiagnosis.di_patient;
exception when others then
dbms_output.put_line('Cannot update Patient di_patient='||di_patient);
raise;
end upd; end if;
return rows_;
end;
/
show errors
declare rows_ number := deleteDiagnosis(456);
begin dbms_output.put_line('deleted '||rows_||' rows'); end;
/
deleted 1 rows
select * from Patient;
PAT_PERSON PAT_SICK
---------- ----------
123 0
An alternative solution, if you prefer (or must) to use a trigger in your application - declare internal function returning count of patient's diagnoses in the trigger body:
create or replace trigger di_patmustbewell
after delete on diagnosis for each row
declare
numdiseases number;
function getNumDiagnosis (di_patient number) return number is
ret number;
pragma autonomous_transaction;
begin
select count(1) into ret from diagnosis where di_patient = getNumDiagnosis.di_patient;
return ret;
end getNumDiagnosis;
begin
numDiseases := getNumDiagnosis(:old.di_patient);
if(numdiseases = 0) then
update patient set pat_sick = 0 where pat_person = :old.di_patient;
end if;
end;
/
show errors;
Trigger DI_PATMUSTBEWELL compiled
Hope it helps you a bit.

Answer 2

You can create a COMPOUND trigger for such cases:
create or replace TRIGGER Di_PatMustBeWell
FOR DELETE ON Diagnosis
COMPOUND TRIGGER
TYPE Di_Patient_Table_type IS TABLE OF DiagnosisCount.Di_Patient%TYPE;
Di_Patient_Table Di_Patient_Table_type;
BEFORE STATEMENT IS
BEGIN
Di_Patient_Table := Di_Patient_Table_type();
END BEFORE STATEMENT;
BEFORE EACH ROW IS
BEGIN
Di_Patient_Table.EXTEND;
Di_Patient_Table(Di_Patient_Table.LAST) := :OLD.Di_Patient;
END BEFORE EACH ROW;
AFTER STATEMENT IS
BEGIN
FOR i IN Di_Patient_Table.FIRST..Di_Patient_Table.LAST LOOP
SELECT NUMDISEASES INTO Numdiseases
FROM DiagnosisCount
where Di_Patient = Di_Patient_Table(i);
IF NumDiseases != 1 THEN
UPDATE Patient SET Pat_Sick = 0 WHERE Pat_Person = Di_Patient_Table(i);
END IF;
END LOOP;
Di_Patient_Table.DELETE;
END AFTER STATEMENT;
END;
/