Show the player ids of the players in the team Boston Red Stockings in 2001.
select player_id from salary as t1 join team as t2 on t1.team_id = t2.team_id_br where t2.name = 'Boston red Stockings' and t1.year = 2001 group by player_id
How about in 2000?
select player_id from salary as t1 join team as t2 on t1.team_id = t2.team_id_br where t2.name = 'Boston red Stockings' and t1.year = 2000 group by player_id
How many players were there that year?
select count(*) from salary as t1 join team as t2 on t1.team_id = t2.team_id_br where t2.name = 'Boston red Stockings' and t1.year = 2000 group by player_id