DB2: How to join indirectly referenced data - db2

Question

I have the following given table structure (I've removed some columns and created a stub) to support versioning and reduce duplication of data. Imagine an article review process whereas each step is stored in database (article_meta). Whenever the article itself changes, the data is stored in DB, too.
The versioning is done by a reference to the predecessor (pre_meta_id).
WITH
t_article_meta (id, pre_meta_id, user_id, state) as (
values (1, NULL, 101, 'submitted')
union all values (2, 1, 7, 'inreview')
union all values (3, 2, 7, 'rejected')
union all values (4, 3, 101, 'submitted')
union all values (5, NULL, 202, 'submitted')
union all values (6, 5, 7, 'inreview')
union all values (7, 6, 7, 'accepted')
union all values (8, 4, 7, 'inreview')
union all values (9, 8, 7, 'accepted')
),
t_article (id, meta_id, content) as (
values (1, 1, 'Hello wordl')
union all values (2, 4, 'Hello world')
union all values (3, 5, 'Lorem ipsum doloret')
)
SELECT ...;
Now I want to create a view that somehow combines meta data and article data even if there is no direct reference (only indirect via predecessor).
id | pre_meta_id | user_id | state | content (left join) | content (I want to have)
---|-------------|---------|-----------|---------------------|-------------------------
1 | NULL | 101 | submitted | Hello wordl | Hello wordl
2 | 1 | 7 | inreview | NULL | Hello wordl
3 | 2 | 7 | rejected | NULL | Hello wordl
4 | 3 | 101 | submitted | Hello world | Hello world
5 | NULL | 202 | submitted | Lorem ipsum doloret | Lorem ipsum doloret
6 | 5 | 7 | inreview | NULL | Lorem ipsum doloret
7 | 6 | 7 | accepted | NULL | Lorem ipsum doloret
8 | 4 | 7 | inreview | NULL | Hello world
9 | 8 | 7 | accepted | NULL | Hello world
How can I realize something like that in DB2 in a performing way? My first idea: a join on a function (to get the predecessor with an article related) sounds really expensive to me.

Answer 1

This SQL would do the job:
SELECT m.id, successor_id, user_id, state, content,
last_value(content,'IGNORE NULLS') over (order by m.id) as last_value
FROM article_meta m
LEFT JOIN article a
ON m.id = a.article_meta_id
ORDER BY m.id
It is the regular join to combine the tables with an aditional column (with another name compared to your expected result to show the difference)
You might want to rename that column and remove content to get a exact match to you expected result.
For the adjusted requirements the SQL gets more complex as we have to define a recursive query to get the title/content for all the childs - it will look like this:
with temp (id, pre_meta_id, user_id, state, level, parent, root) as (
select m.id, m.pre_meta_id, m.user_id, m.state, 1 as level, m.pre_meta_id as parent, m.id as root
from article_meta m, article a
where m.id = a.meta_id
union all
select m.id, m.pre_meta_id, m.user_id, m.state, level + 1 as level, t.id as parent, t.root
from temp t, article_meta m
where m.pre_meta_id = t.id
and m.id not in (select meta_id from article)
and level < 10
)
select *
from temp t
left join article a
on t.root = a.meta_id
order by 1