Query for Getting previous date in oracle in specifc scenario

Question

I have the below data in a table A which i need to insert into table B along with one compute column.
TABLE A:
Account_No | Balance | As_on_date
1001 |-100 | 1-Jan-2013
1001 |-150 | 2-Jan-2013
1001 | 200 | 3-Jan-2013
1001 |-250 | 4-Jan-2013
1001 |-300 | 5-Jan-2013
1001 |-310 | 6-Jan-2013
Table B:
In table B,there should be no of days to be shown when balance is negative and the date one which it has gone into negative.
So,for 6-Jan-2013,this table should show below data:
Account_No | Balance | As_on_date | Days_passed | Start_date
1001 | -310 | 6-Jan-2013 | 3 | 4-Jan-2013
Here,no of days should be the days when the balance has gone negative in recent time and not from the old entry.
I need to write a sql query to get the no of days passed and the start date from when the balance has gone negative.
I tried to formulate a query using Lag analytical function,but i am not getting how should i check the first instance of negative balance by traversing back using LAG function. Even the first_value function was given a try but not getting how to partition in it based on negative value.
Any help or direction on this will be really helpful.
Thanks

Answer 1

I think you can apply the Tabibitoshan method here. The query is more complex but performs better than other methods. For an explanation, see Tabibitosan method tutorial by Aketi Jyuuzou
The idea behind the method is to identify "continuous" records without gaps. Here I identify records that are "continuous" because they contain an uninterrupted series of negative balances.
drop table t;
CREATE TABLE t (Account_No , Balance , As_on_date) AS SELECT
1001 ,-100 , to_date('1-Jan-13', 'DD-MON-YY') FROM dual UNION ALL SELECT
1001 ,-150 , to_date('2-Jan-13', 'DD-MON-YY') FROM dual UNION ALL SELECT
1001 , 200 , to_date('3-Jan-13', 'DD-MON-YY') FROM dual UNION ALL SELECT
1001 ,-250 , to_date('4-Jan-13', 'DD-MON-YY') FROM dual UNION ALL SELECT
1001 ,-300 , to_date('5-Jan-13', 'DD-MON-YY') FROM dual UNION ALL SELECT
1001 ,-310 , to_date('6-Jan-13', 'DD-MON-YY') FROM dual;
SELECT Account_No, Balance, to_char(As_on_date, 'DD-MON-YYYY') as_on_date,
CASE
  WHEN balance < 0
  THEN row_number() OVER(PARTITION BY account_no, grp ORDER BY as_on_date)
END days_passed,
CASE
  WHEN balance < 0
  THEN to_char(
    first_value(as_on_date) OVER(PARTITION BY account_no, grp ORDER BY as_on_date),
    'DD-MON-YYYY'
end start_date
from (
  SELECT Account_No, Balance, As_on_date,
  CASE WHEN balance >= 0 THEN NULL ELSE
    row_number() OVER(PARTITION BY account_no ORDER BY as_on_date) -
    sum(CASE WHEN balance < 0 THEN 1 ELSE 0 END) OVER(PARTITION BY account_no ORDER BY as_on_date)
  end grp
  FROM t
order by account_no, As_on_date;
ACCOUNT_NO
BALANCE
AS_ON_DATE
DAYS_PASSED
START_DATE
1001
-100
01-JAN-2013
1
01-JAN-2013
1001
-150
02-JAN-2013
2
01-JAN-2013
1001
200
03-JAN-2013
1001
-250
04-JAN-2013
1
04-JAN-2013
1001
-300
05-JAN-2013
2
04-JAN-2013
1001
-310
06-JAN-2013
3
04-JAN-2013