Recursive SELECT with stop condition in SQL? - sql

Question

My table called element looks like this:
id | successor | important
----------------------------
1 | NULL | 0
2 | 4 | 1
3 | 5 | 0
4 | 8 | 0
5 | 6 | 1
6 | 7 | 0
7 | NULL | 0
8 | 10 | 1
9 | 10 | 0
10 | NULL | 0
I start with an element’s ID. Each element may or may not have a succeeding element. So given any element ID I may build a chain of elements from 0..n elements depending on its successors and successor-successors, and so on.
Let’s say my starting ID is 2. This results in the following chain:
2 -> 4 -> 8 -> 10
Now I want to ask this question: Does a specific element chain contain at least one element where important == 1?
In pseudo-code a function realizing this without unneccessary checks may look like this:
boolean chainIsImportant(element)
{
if (element.important == 1) {
return true;
}
if (element.successor != NULL) {
return chainIsImportant(element.successor);
}
return false;
}
I guess this can be realized with WITH RECURSIVE, right? How can I stop recursion, once an element with important == 1 was found?

Answer 1

This is typically done by aggregating the columns in question and adding a condition on the join in the recursive part of the CTE:
with recursive all_elements as (
select id, successor, important, array[important] as important_elements
from elements
where successor is null
union all
select c.id, c.successor, c.important, p.important_elements||c.important
from elements c
join all_elements p on c.successor = p.id
where 1 <> all(p.important_elements)
)
select *
from all_elements;
Note that the condition is "flipped" because the where clause defines those rows that should be included.

Answer 2

CREATE TABLE booltree
( id INTEGER NOT NULL PRIMARY KEY
, successor INTEGER REFERENCES booltree(id)
, important Boolean NOT NULL
);
INSERT INTO booltree(id , successor , important) VALUES
( 1, NULL , False)
,(2, 4 , True)
,(3, 5 , False)
,(4, 8 , False)
,(5, 6 , True)
,(6, 7 , False)
,(7, NULL , False)
,(8, 10 , True)
,(9, 10 , False)
,(10, NULL , False)
;
-- SELECT * FROM booltree;
WITH RECURSIVE rec AS (
SELECT id, important
FROM booltree
WHERE successor IS NULL
UNION ALL
SELECT bt.id, GREATEST(rec.important, bt.important) AS important
FROM booltree bt
JOIN rec ON bt.successor = rec.id
)
SELECT id, important
FROM rec
ORDER BY important, id;
Result:
CREATE TABLE
INSERT 0 10
id | important
----+-----------
1 | f
6 | f
7 | f
9 | f
10 | f
2 | t
3 | t
4 | t
5 | t
8 | t
(10 rows)
Note: IMHO the recursion cannot be stopped once a True importance is found (basically, because LEFT JOINS are not allowed in RECURSIVE UNIONS)
But if you are looking for exactly one given id (or a set of them) then maybe you could use that as the start condition, and search the tree upwards.