小编use*_*072的帖子

我必须编写一个查询来计算每个客户购物之间的平均天数(不使用子查询).

    create table data {
customer varchar(20) not null,
bought date not null,
primary key (customer,bought)
}

例如,

insert into data (customer,bought)
values (‘John Smith’, date ‘2011-02-01’),
(‘Alice Cooper’, date ‘2011-02-01’),
(‘Bob Baker’, date ‘2011-02-01’),
(‘John Smith’, date ‘2011-02-02’),
(‘Bob Baker’, date ‘2011-02-02’),
(‘Bob Baker’, date ‘2011-02-03’),
(‘Bob Baker’, date ‘2011-02-04’),
(‘Bob Baker’, date ‘2011-02-05’),
(‘Bob Baker’, date ‘2011-02-06’),
(‘Bob Baker’, date ‘2011-02-07’),
(‘John Smith’, date ‘2011-02-07’),
(‘Alice Cooper’, date ‘2011-02-08’);

应该回答约翰史密斯等了1天然后5天,所以他的平均值是3天.Alice Cooper(!)等了7天,所以她的平均值是7.Bob Baker是每日跑步者,所以他的平均值是1.

我做过这样的事情

select distinct customer, avg (bought) …

我必须与一些本科生和研究生一起创建课程,然后使用方法getPostgraduates()从课程中提取所有以"Ismael Bento"为主管的研究生,并使用类Notifier向他们发送消息(打印文本和接受者).然而,没有任何东西被打印出来......我的猜测是我的getPostgraduates()方法有问题.

这是主要方法:

package students;

import java.util.*;

public class ProgrammingTest {

    public static void main (String[] args){
        Academic rr = new Academic("Ricardo Rodriguez");
        Academic ib = new Academic("Ismael Bento");
        Set<Student> students = new HashSet<Student>();

        Undergraduate ug1 = new Undergraduate("gg4", "Greg","gg4@", rr);
        Undergraduate ug2 = new Undergraduate("pr3", "Pete","pr3@", ib);
        Postgraduate pg1 = new Postgraduate("te2", "Ted", "te2@", rr);
        Postgraduate pg2 = new Postgraduate("yj34", "Yao", "yj34@", ib);
        Postgraduate pg3 = new Postgraduate("jj8", "Jack", "jj8@", ib);

        students.add(ug1);
        students.add(ug2);
        students.add(pg1);
        students.add(pg2);
        students.add(pg3);

        Course c1 = …