Get rows from current month if older is not available - sql

Question

I have a table that looks like this:
+--------------------+---------+
| Month (date) | amount |
+--------------------+---------+
| 2016-10-01 | 20 |
| 2016-08-01 | 10 |
| 2016-07-01 | 17 |
+--------------------+---------+
I'm looking for a query (sql statement) which satisfies the following conditions:
Give me the value of the previous month.
If there is no value for the previous month lock back in time until one can be found.
If there is just a value for the current month give me this value.
In the example table the row I'm looking for would be this:
+--------------------+---------+
| 2016-08-01 | 10 |
+--------------------+---------+
Has anyone a idea for a non complex select query?
Thanks in advance,
Peter

Answer 1

You may need the following:
SELECT *
FROM ( SELECT *
FROM test
WHERE TRUNC(SYSDATE, 'month') >= month
ORDER BY CASE
WHEN TRUNC(SYSDATE, 'month') = month
THEN 0 /* if current month, ordered last */
ELSE 1 /* previous months are ordered first */
END DESC,
month DESC /* among previous months, the greatest first */
)
WHERE ROWNUM = 1

Answer 2

Another way using MAX
WITH tbl AS (
SELECT TO_DATE('2016-10-01', 'YYYY-MM-DD') AS "month", 20 AS amount FROM dual
UNION
SELECT TO_DATE('2016-08-01', 'YYYY-MM-DD') AS "month", 10 AS amount FROM dual
UNION
SELECT TO_DATE('2016-07-01', 'YYYY-MM-DD') AS "month", 5 AS amount FROM dual
)
SELECT *
FROM tbl
WHERE TRUNC("month", 'MONTH') = NVL((SELECT MAX(t."month")
FROM tbl t
WHERE t."month" < TRUNC(SYSDATE, 'MONTH')),
TRUNC(SYSDATE, 'MONTH'));

Answer 3

I would use row_number():
select t.*
from (select t.*,
row_number() over (order by (case when to_char(dte, 'YYYY-MM') = to_char(sysdate, 'YYYY-MM') then 1 else 2 end) desc,
dte desc
) as seqnum
from t
) t
where seqnum = 1;
Actually, you don't need row_number() for this:
select t.*
from (select t.*
from t
order by (case when to_char(dte, 'YYYY-MM') = to_char(sysdate, 'YYYY-MM') then 1 else 2 end) desc,
dte desc
) t
where rownum = 1;

Answer 4

It's not the nicest query but it should work.
select amount, date from (
select amount, date, row_number over(partition by HERE_PUT_ID order by
case trunc(date, 'month') when trunc(sysdate, 'month') then to_date('00010101', 'yyyymmdd') else trunc(date, 'month') end
desc) r)
where r = 1;
I guess you have some id in table so put id column instead of HERE_PUT_ID if you want query for whole table just delete: partition by HERE_PUT_ID

Answer 5

I added more data for testing, and an "id" column (a more realistic scenario) to show how this would work. If there is no "id" in your data, simply delete any reference to it from the solution.
Notes - month is a reserved Oracle word, don't use it as a column name. The solution assumes the date column contains dates that are already truncated to the beginning of the month. The trick in "order by" in the dense_rank last is to assign a value (ANY value!) when the month is the current month; by default, the value assigned to all other months is NULL, which by default come after any non-null value in an ascending order.
You may want to test the various solutions for efficiency if execution time is important.
with
inputs ( id, mth, amount ) as (
select 1, date '2016-10-01', 20 from dual union all
select 1, date '2016-08-01', 10 from dual union all
select 1, date '2016-07-01', 17 from dual union all
select 2, date '2016-10-01', 30 from dual union all
select 2, date '2016-09-01', 25 from dual union all
select 3, date '2016-10-01', 20 from dual union all
select 4, date '2016-08-01', 45 from dual union all
select 4, date '2016-06-01', 30 from dual
)
-- end of TEST DATA - the solution (SQL query) is below this line
select id,
max(mth) keep(dense_rank last order by
case when mth = trunc(sysdate, 'mm') then 0 end, mth) as mth,
max(amount) keep(dense_rank last order by
case when mth = trunc(sysdate, 'mm') then 0 end, mth) as amount
from inputs
group by id
order by id -- ORDER BY is optional
;
ID MTH AMOUNT
--- ---------- -------
1 2016-08-01 10
2 2016-09-01 25
3 2016-10-01 20
4 2016-08-01 45

Answer 6

You could sort the data in the direction you want to:
with MyData as
(
SELECT to_date('2016-10-01','YYYY-MM-DD') MY_DATE, 20 AMOUNT FROM DUAL UNION
SELECT to_date('2016-08-01','YYYY-MM-DD') MY_DATE, 10 AMOUNT FROM DUAL UNION
SELECT to_date('2016-07-01','YYYY-MM-DD') MY_DATE, 17 AMOUNT FROM DUAL
),
MyResult AS (
SELECT
D.*
FROM MyData D
ORDER BY
DECODE(
12*TO_CHAR(MY_DATE,'YYYY') + TO_CHAR(MY_DATE,'MM'),
12*TO_CHAR(SYSDATE,'YYYY') + TO_CHAR(SYSDATE,'MM'),
-1,
12*TO_CHAR(MY_DATE,'YYYY') + TO_CHAR(MY_DATE,'MM'))
DESC
)
SELECT * FROM MyResult WHERE RowNum = 1