Order by in subquery and alias - sql

Question

I have a problem with a order by in oracle query.
select KEY, B, C, (select D from TABLE1 a where a.KEY = b.KEY and a.DATE<
b.DATE order BY a.DATE and rownum =1 )
FROMSTATUS from TABLE2 b
I known the "order by" is not working in subquery. I modify my query as:
select KEY, B, C, (select * from (select D from TABLE1 a where a.KEY =
b.KEY and a.DATE< b.DATE order by DATE) where rownum = 1)
FROMSTATUS from TABLE2 b
But in this way the B.KEY and B.DATE has not resolved by oracle
I need select only a 1 value from TABLE2 and the value is the first previous a.DATE
Example:
TABLE1
KEY DATE A B C
1 01/31/2000 1 2 3
2 02/25/2000 X Y Z
TABLE2
KEY DATE D
1 01/30/2000 1
1 01/27/2000 2
1 01/25/2000 2
2 02/20/2000 4
2 02/13/2000 1
I need this result:
TABLE1.KEY TABLE1.DATE TABLE1.A TABLE1.B TABLE1.C TABLE2.DATE TABLE2.D
1 01/31/2000 1 2 3 01/30/2000 1
2 02/25/2000 X Y Z 02/20/2000 4
Can you help me?
(i am sorry for my bad english)

Answer 1

row_number() after union will get your output.
select tFinal.DATE, tFinal.KEY
from (select row_number() over (partition by KEY order by t1.T, t1.DATE desc) as rn, t1.DATE, t1.KEY
from
(select DATE, KEY, 't1' as T from TABLE1
union all
select DATE, KEY, 't2' as T from TABLE2) t1) tFinal
Where rn = 2

Answer 2

You can use window functions for this:
WITH cte AS (
SELECT TABLE2.KEY, TABLE2.B, TABLE2.C, TABLE1.D
, ROW_NUMBER() OVER (PARTITION BY TABLE2.KEY, TABLE2.DATE ORDER BY TABLE1.DATE DESC) AS rn
FROM TABLE2
LEFT JOIN TABLE1 ON TABLE2.KEY = TABLE1.KEY AND TABLE2.DATE > TABLE1.DATE
)
SELECT *
FROM cte
WHERE rn = 1

Answer 3

Here's an answer that uses aggregation:
WITH t1 AS (SELECT 1 KEY, to_date('31/01/2000', 'dd/mm/yyyy') dt FROM dual UNION ALL
SELECT 2 KEY, to_date('25/02/2000', 'dd/mm/yyyy') dt FROM dual),
t2 AS (SELECT 1 KEY, to_date('30/01/2000', 'dd/mm/yyyy') dt FROM dual UNION ALL
SELECT 1 KEY, to_date('27/01/2000', 'dd/mm/yyyy') dt FROM dual UNION ALL
SELECT 1 KEY, to_date('25/01/2000', 'dd/mm/yyyy') dt FROM dual UNION ALL
SELECT 2 KEY, to_date('20/02/2000', 'dd/mm/yyyy') dt FROM dual UNION ALL
SELECT 2 KEY, to_date('13/02/2000', 'dd/mm/yyyy') dt FROM dual)
SELECT t1.KEY,
t1.dt t1_date,
MAX(t2.dt) t2_date
FROM t1
LEFT OUTER JOIN t2 ON t1.key = t2.key AND t2.dt < t1.dt
GROUP BY t1.key, t1.dt
ORDER BY t1.key;
KEY T1_DATE T2_DATE
---------- ----------- -----------
1 31/01/2000 30/01/2000
2 25/02/2000 20/02/2000
I'm assuming here that t1.key is a unique column. Whether this is more performant than any of the other answers for your data is up to you to test *{:-)

Answer 4

In Oracle you can use KEEP LAST for this:
select
key,
b,
c,
(
select max(d) keep (dense_rank last order by t2.date)
from table2 t2
where t2.key = t1.key and t2.date < t1.date
) as fromstatus
from table1 t1;
As of Oracle 12c you can also use FETCH FIRST ROW:
select
key,
b,
c,
(
select d
from table2 t2
where t2.key = t1.key and t2.date < t1.date
order by t2.date desc
fetch first row only
) as fromstatus
from table1 t1;
or, moving the subquery to the FROM clause:
select
t1.key,
t1.b,
t1.c,
first_t2.d as fromstatus
from table1 t1
outer apply
(
select d
from table2 t2
where t2.key = t1.key and t2.date < t1.date
order by t2.date desc
fetch first row only
) first_t2;
This last query has the advantage that you could easily select more values from the table2 row than just one.