Oracle Date column cleaning

Question

I have a table1(DueDate varchar2(20)). It has thousands of date data in different format with some bad data like characters.

eg.

YYYYMMDD,
MM/DD/YYY         


        

Answer 1

Pure SQL solution for Oracle 12.2, where the handy default null on conversion error option avoids the need to write functions and handle exceptions:

with demo (date_string) as
     ( select '2018081' from dual union all
       select '01/8/2018' from dual union all
       select '01-8-2018' from dual union all
       select 'ABCD' from dual union all
       select '2018-08-01' from dual union all
       select '2018-01-8' from dual union all
       select '2017081' from dual union all
       select '01-AUG-2018' from dual
     )
select t.date_string
     , coalesce
       ( to_date(t.date_string default null on conversion error)
       , case sys_context('USERENV','NLS_TERRITORY')
              when 'AMERICA' then to_date(t.date_string default null on conversion error,'MM/DD/YYYY')
              else to_date(t.date_string default null on conversion error,'DD/MM/YYYY')
         end
       , to_date(t.date_string default null on conversion error,'DD/MM/YYYY')
       , to_date(t.date_string default null on conversion error,'YYYY/MM/DD')
       , to_date(t.date_string default null on conversion error,'YYYYMMDD')
       , to_date(t.date_string default null on conversion error,'DD/MON/YYYY')
       ) as converted_date
from   demo t

Results:

DATE_STRING CONVERTED_DATE
----------- --------------
2018081     01-AUG-2018
01/8/2018   08-JAN-2018
01-8-2018   08-JAN-2018
ABCD
2018-08-01  01-AUG-2018
2018-01-8   08-JAN-2018
2017081     01-AUG-2017
01-AUG-2018 01-AUG-2018

The first test does not specify any format in order to use the session default (DD-MON-RRRR etc).

The next test attempts to handle the North American habit of placing the month first. (I'm not sure if sys_context('USERENV','NLS_TERRITORY') = 'AMERICA' is a reliable test though, as that tends to be the default setting regardless of actual territory. However it illustrates how you can inject some conditional logic if desired.)

to_date is pretty forgiving about separator characters so I haven't specified different formats for DD-MON-YYYY vs DD/MON/YYYY etc.