Getting all parent rows in one SQL query

Question

I have a simple MySQL table thats contains a list of categories, level is determined by parent_id:

id  name    parent_id
---------------------------
1   Home         


        

Answer 1

If You have slug instead of id then simply run sub-query to find id of child category.
Table - categories
| id | parentId | slug |
|-------------------------|
| 1  |    0      |    u1    |
| 2  |    1      |    u2    |
| 3  |    2      |    u3    |
| 4  |    0      |    u4    |

SELECT T2.id, T2.slug
FROM (
    SELECT
        @r AS _id,
        (SELECT @r := parentId FROM categories WHERE id = _id) AS parentId,
        @l := @l + 1 AS lvl
    FROM
        (SELECT @r := (SELECT id FROM categories WHERE slug = 'u3'), @l := 0) vars,
        categories h
    WHERE @r <> 0) T1
JOIN categories T2
ON T1._id = T2.id
ORDER BY T1.lvl DESC