Sql question: Getting Parent rows followed by child rows

Question

id parent_id

1 0

2 0

3 2

4 0

5 1

6 0

I need a query that will return parent rows (parent_id=0) f

Answer 1

Here is an example solution using a union with an order by clause (It wont work for deep nesting though).

SELECT  p.id, 
        p.parent_id, 
        p.name,  
        p.id AS sequence
FROM topics AS p 
WHERE p.parent_id = 0
UNION 
SELECT  t.id, 
        t.parent_id, 
        t.name, 
        t.parent_id AS sequence
FROM topics AS t
WHERE t.parent_id <> 0
ORDER BY sequence, parent_id, name