How many people have ever been on the negative side of debates?
select count(distinct t2.people_id) from debate_people as t1 join people as t2 on t1.negative = t2.people_id
How many times have they been on the negative side of debates?
select t2.name, count(*) from debate_people as t1 join people as t2 on t1.negative = t2.people_id group by t2.name
How many of them have been on the negative side of debates at least twice?
select count(distinct t2.name) from debate_people as t1 join people as t2 on t1.negative = t2.people_id group by t2.name having count(*) >= 2
Show their names.
select t2.name from debate_people as t1 join people as t2 on t1.negative = t2.people_id group by t2.name having count(*) >= 2