if (grep {$_ eq 'experience.country'} ($self->{field1}, $self->{field2}, $self->{field3})) {
        $sqlquery .= q#
        JOIN has_regional_experience ON people.ID_person = has_regional_experience.ID_person
        JOIN countries AS experience ON has_regional_experience.ID_country = experience.ID_country        
        #;
    }

    if (grep {$_ eq 'nationality.country'} ($self->{field1}, $self->{field2}, $self->{field3})) {
        $sqlquery .= q#
        JOIN belongs_to ON people.ID_person = belongs_to.ID_person
        JOIN countries AS nationality ON belongs_to.ID_country = nationality.ID_country        
        #;
    }
    
    if (grep {$_ eq 'expertise'} ($self->{field1}, $self->{field2}, $self->{field3})) {
        $sqlquery .= q#
        JOIN owns ON people.ID_person = owns.ID_person
        JOIN expertise ON owns.ID_expertise = expertise.ID_expertise        
        #;
    }

    if (grep {$_ eq 'language'} ($self->{field1}, $self->{field2}, $self->{field3})) {
        $sqlquery .= q#
        JOIN spoke ON people.ID_person = spoke.ID_person
        JOIN languages ON spoke.ID_language = languages.ID_language        
        #;
    }

    if (grep {$_ eq 'donor'} ($self->{field1}, $self->{field2}, $self->{field3})) {
        $sqlquery .= q#
        JOIN worked_for ON people.ID_person = worked_for.ID_person
        JOIN donors ON worked_for.ID_donor = donors.ID_donor    
        #;
    }