sqlite update a column with itself - sqlite

Question

I got a table like this
a b c
-- -- --
1 1 10
2 1 0
3 1 0
4 4 20
5 4 0
6 4 0
The b column 'points' to 'a', a bit like if a is the parent.
c was computed. Now I need to propagate the parent c value to their children.
The result would be
a b c
-- -- --
1 1 10
2 1 10
3 1 10
4 4 20
5 4 20
6 4 20
I can't make an UPDATE/SELECT combo that works
So far I got a SELECT that procuce the c column I'd like to get
select t1.c from t t1 join t t2 on t1.a=t2.b;
c
----------
10
10
10
20
20
20
But I dunno how to stuff that into c
Thanx in advance
Cheers, phi

Answer 1

You have to look up the value with a correlated subquery:
UPDATE t
SET c = (SELECT c
FROM t AS parent
WHERE parent.a = t.b)
WHERE c = 0;

Answer 2

I finnally found a way to copy back my initial 'temp' SELECT JOIN to table 't'. Something like this
create temp table u as select t1.c from t t1 join t t2 on t1.a=t2.b;
update t set c=(select * from u where rowid=t.rowid);
I'd like to know how the 2 solutions, yours with 1 query UPDATE correlated SELECT, and mine that is 2 queries and 1 correlated query each, compare perf wise. Mine seems more heavier, and less aesthetic, yet regarding perf I wonder.
On the Algo side, yours take care not to copy the parent data, only copy child data, mine copy parent on itself, but that's a nop, yet consuming some cycles :)
Cheers, Phi