mysql - 通过外键从第二个表中获取第一个表中的字段

Question

两个表:类别，以及它们的多对多关系:

categories
id | name
1    first
2    second
3    third

relations
parent | child
1        2
1        3

我怎样才能得到这个结果？:

first | second
first | third

我只能

SELECT c.name, r.child 
FROM categories AS c 
LEFT JOIN relations AS r ON c.id = r.parent

结果是

first | 2
first | 3

那么，我怎样才能在这个表中得到 child 的名字呢？

Answer 1

SELECT c1.name, c2.name
FROM relations AS r
LEFT JOIN categories AS c1 ON r.parent = c1.id
LEFT JOIN categories AS c2 ON r.child = c2.id
WHERE c1.id = 1