Rewrite query in FOR LOOP to single query - sql

Question

Could you please help me to rewrite the following query inside of FOR LOOP query to single query without loop:
for rec in select distinct sp.student_id, v.test_id, v.name, p.version_id from student_pages sp, pages p, versions v where p.id = sp.page_id and v.id = p.version_id order by student_id, test_id, name LOOP
select STRING_AGG(cast(p.index as varchar), ';' ORDER BY p.index) as lost_page_indices into l_lost_page_indices from pages p left join student_pages sp on p.id = sp.page_id and sp.student_id = rec.student_id where p.version_id = rec.version_id and sp.page_id is null;
end loop;
In the final query I need the following fields: sp.student_id, v.test_id, v.name and lost_page_indices.

Answer 1

First cut: Take the query you're looping over, turn it into a subquery, and use it as a join table.
select STRING_AGG(cast(p.index as varchar), ';' ORDER BY p.index)
as lost_page_indices into l_lost_page_indices
from pages p
join (
select distinct sp.student_id, v.test_id, v.name, p.version_id
from student_pages sp
join pages p on p.id = sp.page_id
join versions v on v.id = p.version_id
) as rec on rec.student_id = sp.student_id
left join student_pages sp on p.id = sp.page_id and
sp.student_id = rec.student_id
where p.version_id = rec.version_id and
sp.page_id is null
order by rec.student_id, rec.test_id, rec.name
I reorgnaized the subquery using join syntax for easier reading.
order by cannot be relied on to survive a join, so it moves into the outer query.
There's no group by so I'm not sure if an aggregate function will work.
And, as others have pointed out, string_agg() is not a built in Oracle function. You have to make it yourself.
That can be simplified, there's a lot of redundancy between the two joins. That subquery is putting together student_pages, pages, and versions which can all be done with a normal join. The only thing left is the distinct sp.student_id which can be better done with a group by sp.student_id.
select STRING_AGG(cast(p.index as varchar), ';' ORDER BY p.index)
as lost_page_indices into l_lost_page_indices
from pages p
left join student_pages sp on sp.page_id = p.id
join versions v on v.id = p.version_id
where sp.page_id is null
group by sp.student_id
order by sp.student_id, v.test_id, v.name
I'm not 100% sure that's an equivalent query, but it should get you started. This makes it a lot clearer what the query is doing: find orphaned pages and stick them into a table.