一、创建三张表
①Sailors(sid char(10),sname char(20),rating int,age int),
其中sid是主关键字，sid表示水手的编号，sname表示水手的姓名，rating表示水手的级别，age表示水手的年龄。
②Boats(bid char(10),bname char(20),color char(10)),
其中bid表示船的编号是主关键字，bname是船的名字，color是船的颜色
③Reserves(sid char(10),bid char(10),rdate date),
Reserves中记录水手在哪天定了那只船，其中sid是指向Sailors的外关键字，bid是指向Boats的外关键字,(sid,bid,rdate)合起来构成Reserves的主关键字。
1.查找定了103号船的水手
select s.*, r.bid
from sailors s, reserves r
where s.sid = r.sid and r.bid = '103';
2.查找定了红色船水手的姓名
select s.sname
from sailors s
where s.sid in (select sid from reserves where bid in (select bid from boats where color = 'red'));
3.查找定了红色船而没有定绿色船的水手姓名
select distinct s.sname
from sailors s
where s.sid in (select sid from reserves where bid in (select bid from boats where color = 'red'))
and s.sid not in (select sid from reserves where bid in (select bid from boats where color = 'green'));
4.查找没有定过船的水手信息
select s.*
from sailors s
where s.sid not in (select sid from reserves);
5.查找定过船而没有定过红色船的水手信息
select s.*
from sailors s
where exists (select * from reserves r where s.sid = r.sid)
and not exists (select * from reserves r, boats b where b.bid = r.bid and s.sid = r.sid and b.color = 'red');
6.查找没有定过红色船的水手信息
select s.*
from sailors s
where not exists (select * from reserves r where s.sid = r.sid)
and not exists (select * from reserves r, boats b where b.bid = r.bid and s.sid = r.sid and b.color = 'red');
7.查找定过所有船的水手姓名和编号
select s.sname, s.sid
from sailors s
where not exists (select * from boats b where not exists (select * from reserves r where s.sid = r.sid));
8.查找年龄最大的水手姓名和年龄
select s.sname, s.age
from sailors s
where s.age >= all(select age from sailors);
9.统计水手表中每个级别组的平均年龄和级别组
select s.rating, avg(s.age)
from sailors s
group by s.rating;
10.统计水手表中每个人数不少于2人的级别组中年满18岁水手的平均年龄和级别组
select s.rating, avg(s.age)
from sailors s
where s.age > 18
group by s.rating having count(s.rating) >= 2;
11.统计水手表中每个级别组的人数
select s.rating, count(s.rating)
from sailors s
group by s.rating;
12.统计水手表中人数最少的级别组及人数
select s.rating, count(s.rating)
from sailors s
group by s.rating
having count(s.rating) <= all(select count(rating) from sailors group by rating);
13.查找定过船而没有定过相同的船的水手姓名
select s.sname
from sailors
where s.sid in (select sid from reserves)
and s.sid not in (select sid from reserves group by bid having count(bid) >= 2);
14.将年龄小于30的水手级别+1
update sailors
set rating=rating+1
where age < 30;
15.删除名字叫lubber的水手的定船信息.
delete from reserves where sid = (select sid from sailors where sname = 'lubber');