How select from query inside case in select - sql

Question

I have following query
SELECT DISTINCT v.id,
( CASE
WHEN date IS NULL THEN v.o_id
ELSE (SELECT os
FROM (SELECT o_id1 os
FROM tablet t2
WHERE t2.vso_id = v.id
ORDER BY date ASC)
WHERE rownum = 1)
END ) AS p_o,
v.o_id AS k_o
FROM tablev v
LEFT JOIN tablet t
ON v.id = t.v_id;
Here is what I need:
TableV has distinct v.id values. I need join that table with another (tableT) which has many records for same v.id and then for each distinct v.id:
select O_ID from V
if record for v.id in tableT does not exists(date is null) then
select O_ID from V (same as above)else select o_id1 from T with
minimum date
Also, aditional, is if possible to join o_id from 1. and o_id/o_id1 from2 to another table and write person's name?
I would like, instead of o_id, for each v.id to join selected o_id to another table(tableO) and select name.
So, I need these columns in result: V.id, p_o, k_o
But it would be really great if it looks like: v.id, p_o.name, k_o.name

Answer 1

To answer your first question, assuming that t.vso_id and t.v_id are the same thing (otherwise I might have misunderstood your question), try this query (I use another DBMS, so you might need to adjust the call to COALESCE a little):
SELECT v.id, COALESCE(t.o_id1, v.o_id), v.o_id
FROM tableV v
LEFT JOIN (
(SELECT vso_id, min(date) AS min_date FROM tableT GROUP BY vso_id) min_dates
JOIN tableT t
ON min_dates.vso_id = t.vso_id AND min_dates.min_date = t.date
) ON v.id = t.v_id;
Here's a breakdown:
(SELECT vso_id, min(date) AS min_date FROM tableT GROUP BY vso_id) AS min_dates
selects a smallest date for each vso_id.
(SELECT vso_id, min(date) AS min_date FROM tableT GROUP BY vso_id) AS min_dates
JOIN tableT t
ON min_dates.vso_id = t.vso_id AND min_dates.min_date = t.date
selects one row per vso_id, and that row will correspond to the smallest date that that vso_id has.
Finally, I join tableV to that query. COALESCE(t.o_id1, v.o_id) will return t.o_id if it is not NULL, v.o_id otherwise. You can use CASE instead, but I find COALESCE to be a better fit for this particular purpose.
To answer your second question, you can then JOIN against the table with names like this:
JOIN table_with_names twn1 ON twn1.id = COALESCE(t.o_id1, v.o_id)
JOIN table_with_names twn2 ON twn2.id = v.o_id
and project twn1.name and twn2.name.