Table left join with itself returning NULL - sql

Question

I have a table like
Tdate Symbol new_close
20100110 xxx 1.2
20100111 xxx 1.3
...
20100110 yyy 1.1
20100111 yyy 1.5
Where Tdate was stored as integer and updated per day for each symbol. I want generate a new table from it by subtracting new_close value to its previous one, which looks like this
Tdate Symbol delta
20100110 xxx =1.2-1.2
20100111 xxx =1.3-1.2
...
20100110 yyy =1.1-1.1
20100111 yyy =1.5-1.1
Here is my code
with delta as
( select a.Tdate as TDate, a.Symbol as Symbol,
a.new_close-b.new_close as Pdelta, b.new_close as oldPrice
from ctsWithSplit a left join ctsWithSplit b
on a.TDate-b.TDate=1 and a.Symbol=b.Symbol)
However, in the new generated table, some delta value is NULL, how to fix it please?

Answer 1

Why is TDATE being stored as a number? Your query will fail to find the previous day when the date is the first of the month - eg: 20150201 - 20150131 = 70, whereas there's only a day between 31st Jan and 1st Feb.
Store dates as DATE or TIMESTAMP datatype, and then you give Oracle a chance at getting the date arithmetic correct.
Perhaps you're after something like:
with sample_data as (select to_date('10/01/2010', 'dd/mm/yyyy hh24:mi:ss') tdate, 'xxx' symbol, 1.2 new_close from dual union all
select to_date('11/01/2010', 'dd/mm/yyyy hh24:mi:ss') tdate, 'xxx' symbol, 1.3 new_close from dual union all
select to_date('10/01/2010', 'dd/mm/yyyy hh24:mi:ss') tdate, 'yyy' symbol, 1.1 new_close from dual union all
select to_date('11/01/2010', 'dd/mm/yyyy hh24:mi:ss') tdate, 'yyy' symbol, 1.5 new_close from dual)
select tdate,
symbol,
new_close - lag(new_close, 1, new_close) over (partition by symbol order by tdate) delta
from sample_data;
TDATE SYMBOL DELTA
---------- ------ -----
10/01/2010 xxx 0.0
11/01/2010 xxx 0.1
10/01/2010 yyy 0.0
11/01/2010 yyy 0.4
If you've never worked with analytic functions, then I suggest you look them up - they're incredibly useful and very powerful.
N.B. If you can't convert your TDATE column to be DATE datatype, then you need to convert the column into a date in any queries you run by using to_date().

Answer 2

When your left join doesn't find a matching row in b (i.e. on the first Tdate, or where there isn't a Tdate for the preceding day), then b.new_close will be NULL, as will the result of the subtraction.
Try:
select
a.Tdate as TDate,
a.Symbol as Symbol,
CASE WHEN b.new_close IS NULL THEN 0 ELSE a.new_close-b.new_close END as Pdelta,
b.new_close as oldPrice
from ctsWithSplit a
left join ctsWithSplit b
on a.TDate-b.TDate=1
and a.Symbol=b.Symbol