Join a table that depends on another table - sql

Question

I have a POST table, a CATEGORY table, a TAG table and a MIGTATION_TAG table, I explain the MIGTATION_TAG table contains the movement of the tags between the categories, for example the tag whose ID = 1 belongs to the category whose l 'ID = 10 if I change its category to 12 a line will be added to the MIGTATION_TAG table as follows:
ID 1 TAG_ID 1 CATEGOTY_ID 12
the POST table
id title content tag_id
---------- ---------- ---------- ----------
1 title1 Text... 1
2 title2 Text... 3
3 title3 Text... 1
4 title4 Text... 2
5 title5 Text... 5
6 title6 Text... 4
the CATEGORY table
id name
---------- ----------
1 category_1
2 category_2
3 category_3
the TAG table
id name fist_category_id
---------- ---------- ----------------
1 tag_1 1
2 tag_2 1
3 tag_3 3
4 tag_4 1
5 tag_5 2
the MIGTATION_TAG table
id tag_id category_id
---------- ---------- ----------------
9 1 3
8 5 1
7 1 2
5 3 1
4 2 2
3 5 3
2 3 3
1 1 3
so i would like to know how many posts are registered for each category.
in some cases if there has been no change of category for a tag then it keeps its first category,
I manage to join the TAG table to the POST table via LEFT JOIN but the problem is that the join must depend on the MIGTATION_TAG table which must check if there has been a migration, if so then it must bring me back the last MAX (tag_id ) for each tag ,
here is my query
select category, COUNT(*) AS numer_of_posts
from(
select CATEGORY.name,
case
when POST.tag_id is not null then CATEGORY.name
end as category
from POST
left join TAG ON POST.tag_id = TAG.id
left join (
select id, MAX(tag_id) tag_id
from MIGTATION_TAG
group by id, tag_id
) MIGTATION_TAG
ON TAG.id = MIGTATION_TAG.tag_id
left join CATEGORY on MIGTATION_TAG.category_id = CATEGORY.id
)
GROUP BY category
;
here is the result i want to display with my query
Important ! for the post with id = 6 the tag_id = 4 whish was not changed so it will be using the fist_category_id in TAG table
category numer_of_posts
---------- --------------
category_1 3
category_2 1
category_3 2
Best regards

Answer 1

You can use:
SELECT MAX(c.name) AS category,
COUNT(*)
FROM post p
INNER JOIN tag t
ON (p.tag_id = t.id)
LEFT OUTER JOIN (
SELECT tag_id,
MAX(category_id) KEEP (DENSE_RANK LAST ORDER BY id) AS category_id
FROM migration_tag
GROUP BY tag_id
) m
ON (t.id = m.tag_id)
INNER JOIN category c
ON ( COALESCE(m.category_id, t.first_category_id) = c.id )
GROUP BY c.id
ORDER BY category
Which, for the sample data:
CREATE TABLE POST ( id, title, content, tag_id ) AS
SELECT 1, 'title1', 'Text...', 1 FROM DUAL UNION ALL
SELECT 2, 'title2', 'Text...', 3 FROM DUAL UNION ALL
SELECT 3, 'title3', 'Text...', 1 FROM DUAL UNION ALL
SELECT 4, 'title4', 'Text...', 2 FROM DUAL UNION ALL
SELECT 5, 'title5', 'Text...', 5 FROM DUAL UNION ALL
SELECT 6, 'title6', 'Text...', 4 FROM DUAL;
CREATE TABLE CATEGORY ( id, name ) AS
SELECT 1, 'category_1' FROM DUAL UNION ALL
SELECT 2, 'category_2' FROM DUAL UNION ALL
SELECT 3, 'category_3' FROM DUAL;
CREATE TABLE TAG (id, name, first_category_id) AS
SELECT 1, 'tag_1', 1 FROM DUAL UNION ALL
SELECT 2, 'tag_2', 1 FROM DUAL UNION ALL
SELECT 3, 'tag_3', 3 FROM DUAL UNION ALL
SELECT 4, 'tag_4', 1 FROM DUAL UNION ALL
SELECT 5, 'tag_5', 2 FROM DUAL;
CREATE TABLE MIGRATION_TAG ( id, tag_id, category_id ) AS
SELECT 9, 1, 3 FROM DUAL UNION ALL
SELECT 8, 5, 1 FROM DUAL UNION ALL
SELECT 7, 1, 2 FROM DUAL UNION ALL
SELECT 5, 3, 1 FROM DUAL UNION ALL
SELECT 4, 2, 2 FROM DUAL UNION ALL
SELECT 3, 5, 3 FROM DUAL UNION ALL
SELECT 2, 3, 3 FROM DUAL UNION ALL
SELECT 1, 1, 3 FROM DUAL;
Outputs:
CATEGORY
COUNT(*)
category_1
3
category_2
1
category_3
2
fiddle

Answer 2

One option uses a left join to bring the tag table, and the a lateral join to lookup the latest migration, ifi any. We can then use conditional logic:
select coalesce(t2.category_id, t.first_category_id) category, count(*) number_of_posts
from post p
inner join tag t on t.id = p.tag_id
outer apply (
select mt.category_id
from migration_tag mt
where mt.tag_id = p.tag_id
order by mt.id desc fetch first row only
) t2
group by coalesce(t2.category_id, t.first_category_id)