SQL查询选择仅匹配特定条件的所有行

Question

我正在参加斯坦福大学的在线数据库课程，并被困在练习中。 Here is the database。现在的问题是：

Find names and grades of students who only have friends in the same grade.

这里是我的尝试：

/* Select all highschoolers and their grades... */ 
SELECT h1.name, h1.grade 
FROM Highschooler h1 

/* ...where their grades are equal to all their friends' grades */ 
WHERE h1.grade = ALL(

    /* Select friends' grades */ 
    SELECT h2.grade 
    FROM Highschooler h2 
    JOIN Friend f 
     ON f.ID1 = h1.ID 
     AND f.ID2 = h2.ID 
); 

而且我得到这个错误：

Query failed to execute: near 'ALL': syntax error

我检查我的SQL与online syntax checker，并把它传递。我错过了什么？

Answer 1

为什么不保持它简单和标准，运行在任何sql的风格？你只能通过连接逃脱。

select name, grade 
from Highschooler h1 
inner join friends f 
    on f.id1=h1.id 
inner join Highschooler h2 
    on h2.id=f.id2 
group by 1,2 
having 
sum(case when h1.grade=h2.grade then 1 else 0 end) = count(*) -- nr of friends with same grade equals total nr of friends.