Postgres: Find missing items in a version table - sql

Question

I have a Table in Postgres 12 which tracks what Items i are use in which Versions v:
CREATE TABLE compare_test(v BIGINT, i BIGINT);
With example data:
INSERT INTO compare_test VALUES
(1,21),
(1,22),
(1,23),
(2,21),
(2,22),
(2,23),
(3,21),
(3,22);
I'm trying to create a View that returns:
source_v
target_v
source_i
target_i
1
3
23
null
2
3
23
null
Queries used to compare missing values in two Tables like:
SELECT l.v as source_v, l.i as source_i,
r.v as target_v, r.i as target_i
FROM compare_test l
LEFT JOIN
compare_test r ON r.i = l.i
WHERE r.i IS NULL;
and
SELECT l.v as source_v, l.i as source_i
FROM compare_test l
WHERE NOT EXISTS
(
SELECT i as target_i
FROM compare_test r
WHERE r.i = l.i
)
do not seem to work if the joined Table is the same Table or if more than 2 Versions are in the Table.
I don't have the option to change the Database Structure but I can use plugins.

Answer 1

The solution below gives those results.
It makes re-use of a CTE.
(but somehow I got a feeling that there should exist a more efficient way)
with cte1 as (
SELECT i
, count(*) cnt
, min(v) min_v
, max(v) max_v
FROM compare_test
GROUP BY i
)
, cte2 as
(
select *
from cte1 as c1
where not exists (
select 1
from cte1 c2
where c2.min_v = c1.min_v
and c2.max_v < c1.max_v
)
)
select distinct
t.v as source_v
, c1.max_v as target_v
, c2.i as source_i
, null as target_i
from cte2 c2
left join compare_test t
on t.i = c2.i
left join cte1 c1
on t.v between c1.min_v and c1.max_v
and c1.i != t.i
order by source_v
But if it's not really required to follow the relations, then it becomes really simple.
Then it's just a left join of the existing to all possible combinations.
select distinct
src.v as source_v
, missing.v as target_v
, src.i as source_i
, missing.i as target_i
from
(
select ver.v, itm.i
from (select distinct v from compare_test) as ver
cross join (select distinct i from compare_test) as itm
left join compare_test t
on t.v = ver.v and t.i = itm.i
where t.v is null
) as missing
left join compare_test as src
on src.i = missing.i and src.v != missing.v
order by target_i, target_v, source_v
source_v | target_v | source_i | target_i
-------: | -------: | -------: | -------:
1 | 5 | 21 | 21
2 | 5 | 21 | 21
3 | 5 | 21 | 21
1 | 5 | 22 | 22
2 | 5 | 22 | 22
3 | 5 | 22 | 22
1 | 3 | 23 | 23
2 | 3 | 23 | 23
1 | 5 | 23 | 23
2 | 5 | 23 | 23
5 | 1 | 44 | 44
5 | 2 | 44 | 44
5 | 3 | 44 | 44
db<>fiddle here