供参考
a.
ActorID is Actor primary key,
PlayID is Play primary key,
ActorID , PlayId is Role Foreign Keys
b.
select a.Name from Actor a left join Role b on a.ActorID=b.ActorID
where b.Character='Elektra'
c.
select a.Name , count(c.Title) as PlayNumbers from Actor a left join Role b on a.ActorID=b.ActorID
left join Play c on b.PlayID=c.PlayID
where a.YearBorn=1978
d.
select c.Title from Actor a left join Role b on a.ActorID=b.ActorID
left join Play c on b.PlayID=c.PlayID
where b.Character in ('Juliet','Tybalt')
e.
select a.Name,c.Title from Actor a left join Role b on a.ActorID=b.ActorID
left join Play c on b.PlayID=c.PlayID
group by a.Name,c.Title
having count(b.Character)>1