Question

我有这个问题要解决：如果两个学生A和B是朋友，而A喜欢B但反之亦然，请删除喜欢条目。

对于后台，Friend表有两列，STU1和STU2。如果他们是朋友，那么将会有一个显示STU1，STU2和STU2，STU1的条目。

在Likes表中，如果学生A喜欢学生B，则会有STU1，STU2的条目，但如果学生B不喜欢学生A，则不会有STU2，STU1的条目。

所以，这就是我尝试过的。问题是它仍然在Likes表中留下两行应该在那里。关于如何解决这个问题的任何想法？

delete from Likes
where exists 
    (select F.STU1, F.STU2 from Friend F
        where exists  
        (select L.STU1, L.STU2 from Likes L, Friend F where
            F.STU1 = L.STU1 and F.STU2 = L.STU2)
        )
    and not exists
        (select L.STU1, L.STU2 from Likes L, Friend F where
            F.STU1 = L.STU2 and F.STU2 = L.STU1)

Answer 1

编辑：

with onewayfriends as (
select f.* from friend f
left outer join likes l1 on l1.stu1=f.stu1 and l1.stu2=f.stu2
left outer join likes l2 on l2.stu1=f.stu2 and l2.stu2=f.stu1
where l1.stu1 is null or l2.stu1 is null)

delete l from likes l, onewayfriends f
where l.stu1 in (f.stu1, f.stu2) and l.stu2 in (f.stu1, f.stu2)

编辑2，因为它的sqllite将cte重写为嵌套查询，并将join作为'where id in in ..'

重写

delete from likes where rowid in (select l.rowid from
(
  select f.* from friend f
  left outer join likes l1 on l1.stu1=f.stu1 and l1.stu2=f.stu2
  left outer join likes l2 on l2.stu1=f.stu2 and l2.stu2=f.stu1
  where l1.stu1 is null or l2.stu1 is null
) f, likes l
where l.stu1 in (f.stu1, f.stu2) and l.stu2 in (f.stu1, f.stu2))

fiddle

Answer 2

你说Friend表有两对。因此，我们需要从Likes表中删除该对，其中该对位于Friend表中，但Likes表中不存在反对。

DELETE
FROM Likes
WHERE EXISTS (SELECT 1 FROM Friend f WHERE Likes.STU1 = f.STU1 AND Likes.STU2 = f.STU2)
  AND NOT EXISTS (SELECT 1 FROM Likes ml WHERE Likes.STU1 = ml.STU2 AND Likes.STU2 = ml.STU1)

SQL子查询没有返回我所期望的

2 个答案: