SQL - 查找只喜欢同年级学生的年级

Question

我正在做一个免费的斯坦福在线课程（这很酷，你应该检查一下）并且我已经绞尽脑汁了至少 2 天，但找不到以下问题的答案。 请帮忙。

问题 4 查找只有同年级朋友的学生的姓名和年级。 返回按年级排序的结果，然后按每个年级内的名称排序。

当我终于想到我得到答案时，我的查询返回了 Friend 表中的所有值。

这是我能想到的最好的。

select h1.id, h1.name, h1.grade, h2.id, h2.name, h2.grade
from friend f1
join highschooler h1 on f1.id1 = h1.id
join highschooler h2 on f1.id2 = h2.id
where h1.grade = any (select h3.grade from friend f2
                    join highschooler h3 on f2.id1 = h3.id
                    where h3.id = f1.id1)

我需要在 SQL Lite 中运行查询。 我正在使用http://sqlfiddle.com来测试我在 SQL Lite 中的查询，这是我正在使用的示例数据。

/* Create the schema for our tables */
create table Highschooler(ID int, name text, grade int);
create table Friend(ID1 int, ID2 int);
create table Likes(ID1 int, ID2 int);

/* Populate the tables with our data */
insert into Highschooler values (1510, 'Jordan', 9);
insert into Highschooler values (1689, 'Gabriel', 9);
insert into Highschooler values (1381, 'Tiffany', 9);
insert into Highschooler values (1709, 'Cassandra', 9);
insert into Highschooler values (1101, 'Haley', 10);
insert into Highschooler values (1782, 'Andrew', 10);
insert into Highschooler values (1468, 'Kris', 10);
insert into Highschooler values (1641, 'Brittany', 10);
insert into Highschooler values (1247, 'Alexis', 11);
insert into Highschooler values (1316, 'Austin', 11);
insert into Highschooler values (1911, 'Gabriel', 11);
insert into Highschooler values (1501, 'Jessica', 11);
insert into Highschooler values (1304, 'Jordan', 12);
insert into Highschooler values (1025, 'John', 12);
insert into Highschooler values (1934, 'Kyle', 12);
insert into Highschooler values (1661, 'Logan', 12);

insert into Friend values (1510, 1381);
insert into Friend values (1510, 1689);
insert into Friend values (1689, 1709);
insert into Friend values (1381, 1247);
insert into Friend values (1709, 1247);
insert into Friend values (1689, 1782);
insert into Friend values (1782, 1468);
insert into Friend values (1782, 1316);
insert into Friend values (1782, 1304);
insert into Friend values (1468, 1101);
insert into Friend values (1468, 1641);
insert into Friend values (1101, 1641);
insert into Friend values (1247, 1911);
insert into Friend values (1247, 1501);
insert into Friend values (1911, 1501);
insert into Friend values (1501, 1934);
insert into Friend values (1316, 1934);
insert into Friend values (1934, 1304);
insert into Friend values (1304, 1661);
insert into Friend values (1661, 1025);
insert into Friend select ID2, ID1 from Friend;

insert into Likes values(1689, 1709);
insert into Likes values(1709, 1689);
insert into Likes values(1782, 1709);
insert into Likes values(1911, 1247);
insert into Likes values(1247, 1468);
insert into Likes values(1641, 1468);
insert into Likes values(1316, 1304);
insert into Likes values(1501, 1934);
insert into Likes values(1934, 1501);
insert into Likes values(1025, 1101);

先感谢您。

问候。

塞萨尔

Answer 1

所以我们想找到那些没有其他年级学生的学生，他们有友谊关系，对吧？ 这是表达这一点的一种方式：

select * from highschooler h
where not exists
(select 1 from highschooler h2 where h2.grade != h.grade and exists
(select 1 from friends f where (f.id1 = h.id or f.id2 = h.id) and (f.id1 = h2.id or f.id2 = h2.id)))
order by grade, name

编辑：如果你还要求他们至少有一个朋友，你也需要检查一下

Answer 2

我的解决方案

SELECT name, grade
FROM Highschooler
WHERE ID NOT IN
(SELECT ID1
FROM Friend F1 JOIN Highschooler H1
ON H1.ID = F1.ID1
JOIN Highschooler H2
ON H2.ID = F1.ID2
WHERE H1.grade <> H2.grade)
ORDER BY grade, name

本质上，内部子查询返回学生与具有不同成绩的朋友的关系（其中H1.grade <>到H2.grade）。 然后外部查询只列出所有没有这种内在关系特征的学生。

Answer 3

我更喜欢这个解决方案，因为它只使用在课程中看到的内容，并且如果有没有朋友的学生也可以使用。

select distinct name, grade
from highschooler h join friend f on h.id = f.id1
where id not in
    (select id1
    from friend f join highschooler h2 on h2.id = f.id2
    where h.grade <> h2.grade)
order by grade, name;

关于这个问题需要注意的一件事是关于 Friend 表的规则。 友谊是相互的，所以如果 (123, 456) 在 Friend 表中，那么 (456, 123) 也是。