list the authors have submissions
select author from submission
show me the list of authors who have submissions to workshop.
select distinct t2.author from acceptance as t1 join submission as t2 on t1.submission_id = t2.submission_id
list the authors who have submissions to more than one workshop.
select t2.author from acceptance as t1 join submission as t2 on t1.submission_id = t2.submission_id group by t2.author having count ( distinct t1.workshop_id ) > 1