do not test it on your database, just think about it.
suppose you run the following tsql on sqlserver or sybase:

create table dropit (col1 varchar(9),col2 varchar(9))
select * from dropit
drop table dropit
create table aa (col1 varchar(9),col3 varchar(9))
insert into dropit values ('1','1')
insert aa values ('1','a')
insert aa values ('1','aa')
select * from aa
update dropit set col2=a.col3 from dropit d,aa a where d.col1=a.col1
select col2 from dropit

what the result you expect for dropit.col2? a or aa