Oracle SQL query previous row result - sql

Question

I am just stuck now, now getting the logic to solve this query. Find the below tables and the output. There is Table A and table B which matches two column ID and DATE. If Date got matched then it should multiply the qty with percent else it should pick previous percent.
Table A Table B
ID Date Percent ID Date Qty
A 01/01/17 0.5 A 01/01/17 10
A 04/01/17 1 A 02/01/17 20
A 06/01/17 2 A 03/01/17 30
B 02/01/17 5 A 05/01/17 40
B 05/01/17 10 A 06/01/17 50
A 07/01/17 60
A 08/01/17 40
B 01/01/17 10
B 02/01/17 50
============================================
column column column comment comment comment
ID Date Qty Previous percent if row not matched
A 01/01/17 0.5 * 10 0.5 got new percent
A 02/01/17 0.5 * 20 0.5
A 03/01/17 0.5 * 30 0.5
A 04/01/17 1* 0 1 got new percent but no qty found
A 05/01/17 1 * 40 1
A 06/01/17 2 * 50 2 got new percent
B 01/01/17 10 * 0 0 no percent found
B 02/02/17 5 * 10 5 got new percent
B 5/1/17 10 * 0 10 got new percent

Answer 1

I interpreted the problem to mean "add a few more columns to Table B" - to show the "most current" percentage, showing its date from Table A, and the "gross quantity" from Table B and the "net quantity" by multiplying by the proper percentage.
If you also need one row for every row in Table A, simply delete the WHERE clause from the outermost query (towards the bottom of the query).
with table_a ( id, dt, pct ) as (
select 'A', to_date('01/01/17', 'mm/dd/rr'), 0.5 from dual union all
select 'A', to_date('04/01/17', 'mm/dd/rr'), 1 from dual union all
select 'A', to_date('06/01/17', 'mm/dd/rr'), 2 from dual union all
select 'B', to_date('02/01/17', 'mm/dd/rr'), 5 from dual union all
select 'B', to_date('05/01/17', 'mm/dd/rr'), 10 from dual
), table_b ( id, dt, qty ) as (
select 'A', to_date('01/01/17', 'mm/dd/rr'), 10 from dual union all
select 'A', to_date('02/01/17', 'mm/dd/rr'), 20 from dual union all
select 'A', to_date('03/01/17', 'mm/dd/rr'), 30 from dual union all
select 'A', to_date('05/01/17', 'mm/dd/rr'), 40 from dual union all
select 'A', to_date('06/01/17', 'mm/dd/rr'), 50 from dual union all
select 'A', to_date('07/01/17', 'mm/dd/rr'), 60 from dual union all
select 'A', to_date('08/01/17', 'mm/dd/rr'), 40 from dual union all
select 'B', to_date('01/01/17', 'mm/dd/rr'), 10 from dual union all
select 'B', to_date('02/01/17', 'mm/dd/rr'), 50 from dual
)
-- end of test data (not part of the SQL query); query begins BELOW THIS LINE
select id, qty_date, pct_date, qty as gross_qty, pct, qty * pct as net_qty
from ( select id, dt as qty_date,
last_value(case flag when 0 then dt end ignore nulls)
over (partition by id order by dt) as pct_date,
last_value(pct ignore nulls)
over (partition by id order by dt, flag) as pct,
qty, flag
from ( select id, dt, pct, null as qty, 0 as flag
from table_a
union all
select id, dt, null as pct, qty, 1 as flag
from table_b
)
)
where flag = 1
order by id, qty_date -- if needed
;
Output:
ID QTY_DATE PCT_DATE GROSS_QTY PCT NET_QTY
-- ---------- ---------- ---------- ---------- ----------
A 2017-01-01 2017-01-01 10 .5 5
A 2017-02-01 2017-01-01 20 .5 10
A 2017-03-01 2017-01-01 30 .5 15
A 2017-05-01 2017-04-01 40 1 40
A 2017-06-01 2017-06-01 50 2 100
A 2017-07-01 2017-06-01 60 2 120
A 2017-08-01 2017-06-01 40 2 80
B 2017-01-01 10
B 2017-02-01 2017-02-01 50 5 250
9 rows selected.