How to fetch in subquerys with the ID of the current row? - sql

Question

Hi StackOverflow Community,
I'm very new at SQL and I have a question with a problem that I'm having that I don't know how to fix (Btw I'm new at SQL).
So I have these tables:
TABLE 1 - Users
-ID -NAME
1 User1
2 User2
3 User3
TABLE 2 - JOBS
-ID -JOB -USER -ID_DIRECCTION
1 Football Match User1 1
2 Bascketball Match User3 1
3 Swimming Race User2 2
4 Handball Game User3 2
5 Tennis Match User2 3
TABLE 3 - DIRECCTIONS
-ID -DIRECCTION -NUMBER
1 Fail Street 34
2 Swimming Street 45
3 Fake Street 01
What I want to do is to show the Table of Jobs but adding a new column with a SELECT that shows the last user that have been on the direcction (If no one has been in the direcction the result will be null) and if are multiple users the most recent one.
I'm close to get it with my current query (i think), but the main problem is that when I do the subquery there's a way to fetch the inner query with the ID of the current row where's executing the main query??
SELECT j.ID, j.JOB, j.USER, j.ID_DIRECCTION,
(SELECT j.USER
FROM USERS u INNER JOIN
JOBS j
ON j.USER = u.ID INNER JOIN
DIRECCTIONS d
ON d.ID = j.ID_DIRECCTION
WHERE d.ID = (SELECT d.ID FROM DIRECCTIONS WHERE d.ID = j.ID_DIRECCTION --I want this to be current ID_DIRECCTION row)
ORDER BY j.ID DESC FETCH FIRST 1 ROWS ONLY
) AS LAST_USER
FROM JOBS j;
How is the way to select the ID of the current row to fetch other things in subquerys????
The result should be like:
-ID -JOB -USER -ID_DIRECCTION -LAS_TUSER
1 Football Match User1 1 null*
2 Bascketball Match User3 1 User1*
3 Swimming Race User2 2 null*
4 Handball Game User3 2 User2*
5 Tennis Match User2 3 User3*
But I'm always getting the same value in LAST_USER

Answer 1

It seems you want LAG, but it isn't fully clear why LAS_TUSER = User3 for Tennis Match (job id 5)
SELECT j.*,
LAG(u.name)
over (
PARTITION BY j.id_direcction
ORDER BY j.id )
FROM users u
inner join jobs j
ON j.username = u.name
ORDER BY j.id
Demo

Answer 2

It's unclear from your question whether you really have the user name in the jobs table, or just the ID.
If you do have the name then you have all th einformation you need in the jobs table and you can just use lag().
But if you have the ID then you need to join to the users table, for both the current and lag IDs. You can still get the previous ID the same way:
select id, job, id_user, id_direcction,
lag(id_user) over (partition by id_direcction order by id) as last_id_user
from jobs
but then use that as a CTE or inline view, and join from that:
select j.id, j.job, u.name, j.id_direcction, lu.name as last_name
from (
select id, job, id_user, id_direcction,
lag(id_user) over (partition by id_direcction order by id) as last_id_user
from jobs
) j
join users u on u.id = j.id_user
left join users lu on lu.id = j.last_id_user
With CTEs to provide the sample data, and making assumptions about what those really hold:
-- sample data
with users (id, name) as (
select 1, 'User1' from dual
union all select 2, 'User2' from dual
union all select 3, 'User3' from dual
),
jobs (id, job, id_user, id_direcction) as (
select 1, 'Football Match', 1, 1 from dual
union all select 2, 'Bascketball Match', 3, 1 from dual
union all select 3, 'Swimming Race', 2, 2 from dual
union all select 4, 'Handball Game', 3, 2 from dual
union all select 5, 'Tennis Match', 2, 3 from dual
)
-- actual query
select j.id, j.job, u.name, j.id_direcction, lu.name as last_name
from (
select id, job, id_user, id_direcction,
lag(id_user) over (partition by id_direcction order by id) as last_id_user
from jobs
) j
join users u on u.id = j.id_user
left join users lu on lu.id = j.last_id_user
order by j.id;
ID JOB NAME ID_DIRECCTION LAST_
---------- ----------------- ----- ------------- -----
1 Football Match User1 1
2 Bascketball Match User3 1 User1
3 Swimming Race User2 2
4 Handball Game User3 2 User2
5 Tennis Match User2 3
You don't need to refer to the direcctions table unless you want to get its description. I've included a version that does that in this db<>fiddle.
Like #KaushikNayak, I don't see how or why you would expect to see User3 against the last row, based on the data you provided.