What were the decisions for each player?
select t2.pname, decision from tryout as t1 join player as t2 on t2.pid = t1.pid
Which of the players received a yes?
select t2.pname from tryout as t1 join player as t2 on t2.pid = t1.pid where decision = 'yes'
How many of them exist?
select count(*) from tryout where decision = 'yes'