从第二个表中通过外键从第一个表中获取字段

Question

两个表：类别及其多对多关系：

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，如何在此表中获取子名称？

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

Answer 2

选择categories表两次。

select parent_c.name parent, child_c.name child
from categories parent_c, categories child_c, relations r
where r.parent = parent_c.id and r.child = child_c.id