Question

Greetings, fellow coders!

I have this table that contains categories and subcategories (actually I don’t, but let’s stick to a classic example):

Id     ParentId     Name
1      NULL         A
2      1            B
3      2            C
4      3            D

5      NULL         B
6      5            D

Is there a way for me to get category “D” (id 4) by querying the table for the full path? (see pseudocode below)

SELECT * FROM Categories WHERE FullPath = "A/B/C/D"

// Result:
Id     ParentId     Name
4      3            D

I know that it’s possible to use left joins to get the full path, but how do I write queries to get the leaf node by providing a path?

EDIT (Solution):

Help the help from both van and Eric, this is what I did:

with p as
(
    select
        c.*,
        cast(c.Name as varchar(1024)) as NamePath
    from
        Categories c
    where 
        ParentCategoryId is null
    union all
    select
        c.*,
        cast(p.NamePath + '/' + c.Name as varchar(1024)) as NamePath
    from
        Categories c
        inner join p on
            c.ParentCategoryId = p.CategoryId
)
select Id, Name
from p
where NamePath = 'A/B/C/D'

Thanks guys, both your answers were very helpful! I wish that I was able to mark them both as the solution.

This time I will simply go for the one with the leasts ammount of points (that is van).

Answer 1

With the help of CTE:

WITH CategoriesWithPath (id, parentid, name, path) AS
(
    SELECT  c.*, cast(c.name AS VARCHAR(1024)) AS "path"
    FROM    @Categories c
    WHERE   parentid is null
    UNION ALL
    SELECT  c.*, cast(p."path" + '/' + c.name AS VARCHAR(1024)) AS "path"
    FROM    @Categories c
    INNER JOIN CategoriesWithPath p ON c.parentid = p.id
)
SELECT  id, parentid, name 
FROM    CategoriesWithPath
WHERE   "path" = 'a/b/c/d'

But I would create a view using this CTA, so that you can easily just execute a SELECT from this view based on “path” filter.