> > > diff --git a/sql/sql_yacc.yy b/sql/sql_yacc.yy
> > > index 10535326ce5..545c8f831b9 100644
> > > --- a/sql/sql_yacc.yy
> > > +++ b/sql/sql_yacc.yy
> > > @@ -8190,6 +8195,10 @@ alter_list_item:
> > >            {
> > >              Lex->alter_info.flags|= ALTER_ADD_PERIOD;
> > >            }
> > > +        | ADD period_for_application_time
> >
> > where's ADD PERIOD if_not_exists?
>
> Not very clean with the behavior expected. Consider CREATE TABLE t (s
> date, e date, period for a(s,e));
> Should ALTER TABLE ADD IF NOT EXISTS PERIOD FOR b replace PERIOD FOR a, or
> return ER_MORE_THAN_ONE_PERIOD?

ER_MORE_THAN_ONE_PERIOD of course. It adds a period name `b`, it's a
different period, and there can be only one. So - an error.

If it'd be

  ALTER TABLE ADD IF NOT EXISTS PERIOD FOR a(....)

it'd be a no-op.

I cannot see of a case when ALTER TABLE ADD IF NOT EXISTS should replace stuff.