Sådan undgår du tabelmutationsfejl

Kort svar - ingen trigger, ingen mutering.

Du kan bruge triggeren med pragma autonomous_transaction til optælling af resterende diagnoser for en bestemt patient, men det er ikke en anbefalet måde at gøre dette på. Bedre du opretter en ny funktion eller procedure til at implementere din logik på slettet diagnose. Noget som dette:

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

En alternativ løsning, hvis du foretrækker (eller skal) bruge en trigger i din ansøgning - angiv intern funktion, der returnerer tælling af patientens diagnoser i triggerlegemet:

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

Håber det hjælper dig lidt.