Issue with case when try to modify the siimple query

Question

Hello PL/SQL Gurus/Experts,
I am stuck with an issue (could be simple) but not gettings any headsway -
select 365/day_num_yr Anul_Fact from Date where date_dt = trunc(sysdate -1) value returns is as - 1.46
need to write this in the form of case (as i need to multiply this to some other outer query), i want to store it ina variable and and then use the same in outer one -
but when use the following then it is always returning the value as 0 and not the same output as returned by the above query without case.
select (case when date_dt =trunc(sysdate -1) then 365/day_num_yr else 0 END) Anul_Fact Kindly help, i appericate your time and effort in advance.

Answer 1

Thanks for providing the Drop table and CTAS script. It eased the job of creating sample data;
I only used few data from the table:
drop table T4;
create table T4(day_num_yr, date_dt) as
select 249, to_date('09/05/2012', 'MM/DD/YYYY') from dual union all
select 250, to_date('09/06/2012', 'MM/DD/YYYY') from dual union all
select 251, to_date('09/07/2012', 'MM/DD/YYYY') from dual union all
select 252, to_date('09/08/2012', 'MM/DD/YYYY') from dual;
select date_dt, sysdate - 1 prev_dt, trunc(sysdate - 1) trunc_prev_dt,
       case when date_dt = trunc(sysdate - 1) then 365 / day_num_yr
            else 0
       end num_day
  from t4;
DATE_DT                   PREV_DT                   TRUNC_PREV_DT             NUM_DAY               
05-SEP-12                 06-SEP-12                 06-SEP-12                 0                     
06-SEP-12                 06-SEP-12                 06-SEP-12                 1.46                  
07-SEP-12                 06-SEP-12                 06-SEP-12                 0                     
08-SEP-12                 06-SEP-12                 06-SEP-12                 0See the output. For Date_DT as 6-Sep, i.e. a Day before Today, we are getting the Output, but for other dates we are not.
One another suggestion I would like to make is, (assuming) you are using the Table only for storing the Current day of Year and not including Public Holidays etc. then you can use a 'DDD' format model to get the Current Day of year starting 1 Jan.
Something like below:
select date_dt, sysdate - 1 prev_dt, trunc(sysdate - 1) trunc_prev_dt,
       case when date_dt = trunc(sysdate - 1) then 365 / to_char(date_dt, 'DDD')
            else 0
       end num_day
  from t4;