Question

I have a table with:

Can I with a single query organize it to show:

Thank you.

Answer 1

Yes, you could do this bastardization:

ORDER BY CASE parentid WHEN 0 THEN id ELSE parentid END ASC, id ASC

I say it’s a bastardization because it requires a computation for every single row. It will not be performant. The better way to solve this problem would be to use a better datamodel. Instead of storing just parent ids, you can also store using a Nested Set Model. So basically your datamodel would look like:

id | parentid | name   | left | right
1  | 0        | Peter  | 1    | 4
2  | 0        | Marcus | 5    | 8
3  | 1        | Monica | 2    | 3 
4  | 2        | Sergei | 6    | 7

Now, with that, your order would be reduced to:

ORDER BY `left` ASC

Which would do the exact same thing. Give that nested set article a read. There are better ways to solve the problem than just parentid if you need to get data out either filtered or sorted…