What is the average salary of the players?
select avg(salary) from salary
How about for each team?
select t2.team_id_br, avg(t1.salary) from salary as t1 join team as t2 on t1.team_id = t2.team_id_br group by t2.team_id_br
How about for the team 'Boston Red Stockings' ?
select avg(t1.salary) from salary as t1 join team as t2 on t1.team_id = t2.team_id_br where t2.name = 'Boston red Stockings'