问在SQL select语句中，如何计算行的位置？

EN

create table t (
  visitor_id int(11),
  activity_type varchar(10),
  date date
);

insert into t (visitor_id, activity_type, date) values (1, 'hit',   '2012-1-1');
insert into t (visitor_id, activity_type, date) values (1, 'event', '2012-1-2');
insert into t (visitor_id, activity_type, date) values (2, 'hit',   '2012-1-2');
insert into t (visitor_id, activity_type, date) values (2, 'event', '2012-3-5');
insert into t (visitor_id, activity_type, date) values (2, 'hit',   '2012-3-2');
insert into t (visitor_id, activity_type, date) values (1, 'hit',   '2012-3-5');
insert into t (visitor_id, activity_type, date) values (1, 'hit',   '2012-2-1');

visitor_id, activity_type, Position 
1,          hit,              1 
1,          event,            2 
1,          hit,              3 
1,          hit,              4 
2,          hit,              1 
2,          hit,              2 
2,          event,            3 

select visitor_id, activity_type, 'Position'
from t1
order by visitor_id, date
;