为什么我的SQL查询不起作用？

Question

I am working on the following problem:

For each student A who likes a student B where the two are not friends, find if they have a friend C in common (who can introduce them!). For all such trios, return the name and grade of A, B, and C.

Highschooler ( ID, name, grade ) 
English: There is a high school student with unique ID and a given first name in a certain grade. 

Friend ( ID1, ID2 ) 
English: The student with ID1 is friends with the student with ID2. Friendship is mutual, so if (123, 456) is in the Friend table, so is (456, 123). 

Likes ( ID1, ID2 ) 
English: The student with ID1 likes the student with ID2. Liking someone is not necessarily mutual, so if (123, 456) is in the Likes table, there is no guarantee that (456, 123) is also 

My query is below:

SELECT student_a.name, student_a.grade,
       student_b.name, student_b.grade,
       student_c.name, student_c.grade
FROM Highschooler student_a
    INNER JOIN Likes
        ON (Likes.ID1 = student_a.ID)
    INNER JOIN Highschooler student_b
        ON (Likes.ID2 = student_b.ID)
    INNER JOIN Friend friend_1
        ON (friend_1.ID1 = student_a.ID)
    INNER JOIN Highschooler student_C
        ON (student_c.ID = friend_1.ID2)
    INNER JOIN Friend friend_2
        ON (student_c.ID = friend_2.ID1
            AND student_b.ID = friend_2.ID2)
WHERE student_b.ID <> friend_1.ID2

I get the following:

Andrew 10 Cassandra 9 Gabriel 9 Brittany 10 Kris 10 Haley 10 Austin 11 Jordan 12 Andrew 10 Austin 11 Jordan 12 Kyle 12 Gabriel 11 Alexis 11 Jessica 11

When the actual output should be:

Andrew 10 Cassandra 9 Gabriel 9 Austin 11 Jordan 12 Andrew 10 Austin 11 Jordan 12 Kyle 12

I reviewed my query a ton of times, yet can't point out the edge cases that are messing me up. I know this sort of an ill-posed question. However, I can't seem to pinpoint my error.

Answer 1

Let's see the query: Since "like" is a unidirectional relation, it is accurate to see only a single direction, but since friends is symmetrical, you need to be clever. You are having on clauses in your query, where you handle the friends relation as it was assymetric as well. You assume that student_a.ID = friend_1.ID1 and student_c.ID = friend_1.ID2 if a and c are friends. Why do you consider the other direction impossible? Similarly, you assume that friend_2.ID1 = student_c.ID and friend_2.ID = student_b.ID . Why is the other direction impossible?

At the end you have this:

WHERE student_b.ID <> friend_1.ID2

Where you exclude the case that student_b.ID is the second friend in the first friendship, but you do not exclude that b is the first friend or that a participates in the second friendship.

Suggested solution:

SELECT student_a.name, student_a.grade,
       student_b.name, student_b.grade,
       student_c.name, student_c.grade
FROM Highschooler student_a
    INNER JOIN Likes
        ON (Likes.ID1 = student_a.ID)
    INNER JOIN Highschooler student_b
        ON (Likes.ID2 = student_b.ID)
    INNER JOIN Friend friend_1
        ON (friend_1.ID1 = student_a.ID) OR (friend_1.ID2 = student_a.ID)
    INNER JOIN Highschooler student_C
        ON (student_c.ID = friend_1.ID1) OR (student_c.ID = friend_1.ID2)
    INNER JOIN Friend friend_2
        ON ((student_c.ID = friend_2.ID1 OR student_c.ID = friend_2.ID2)
            AND 
            (student_b.ID = friend_2.ID1 OR student_b.ID = friend_2.ID))

Note, that the query above was not tested and that a where is needed to make sure that a is different from b is different from c is different from a if a student can like himself or be friends with himself.