Get field from first table by foreign key from second

Two tables: categories, and their many-to-many relations:

categories
id | name
1    first
2    second
3    third

relations
parent | child
1        2
1        3

How can I get this result?:

first | second
first | thi开发者_如何转开发rd

I only can

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

And result is

first | 2
first | 3

So, how can I get child name in this table?

---

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

---

Select categories table twice.

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