Question

Have 2 tables for example:

In 1st: object & parent columns

object | parent 
-------+---------
object1| null       
object2| object1  
object3| null  

2nd has: object & reference columns

object | reference
-------+---------
object1| null       
object2| null       
object3| object1       

Need to query tables to order like following: parent is first, then – child(s), objects which have reference(s) to parent.

object1
object2
object3

Is it possible to do in one SQL query or need to sort manually in an array? Seems it is a classical task, probably solution already exists somewhere?

Answer 1

Following recursive query works:

WITH RECURSIVE tables(object, rank) AS (
    SELECT DISTINCT o.object, 1 AS rank FROM oref o
    WHERE o.ref IS NULL
    UNION
    SELECT o.object, t.rank + 1 AS rank
    FROM (SELECT DISTINCT o.object, o.ref FROM oref o
            WHERE ref IS NOT NULL) o, tables t
    WHERE o.ref = t.object AND rank <= t.rank
),
ordered AS (
    SELECT * FROM tables
)
SELECT * FROM tables 
WHERE tables.rank = (SELECT MAX(rank) FROM ordered WHERE ordered.object = tables.object)
ORDER BY rank;

Any comments, questions, objections, propositions? 😉