Retrieve Year from string date in Oracle SQL

Question

I have the following column of dates:

EXPIRY_DAT 31-Oct-17 31-Oct-17 18-Nov-18 12:11:10 31-Dec-12 31-Oct-17 31-Oct-17 18-Nov-18 12:11:10 31-Dec-12 31-Oct-17 31-Oct-17 20-Jul-18 19:20:33 31-Oct-18 31-Oct-18 11-Aug-19 21:52:56 31-Dec-12 . . .

I would like to retrieve the year from these series in the form YYYY. I already tried using the following line of SQL code:

TO_DATE(TO_CHAR(EXPIRY_DAT, 'DD-MON-YY HH:MI:SS'), 'YYYY'

although it returns me the error message:

ORA-01830: date format picture ends before converting entire input string

Answer 1

Use:

select extract(year from TO_DATE(SUBSTR(EXPIRY_DAT,1,9),'DD-Mon-YY')
from your_table

Answer 2

If all your dates are in one of the formats you showed, this could be enough:

-- test case
with dateTable(EXPIRY_DAT) as (
    select '31-Oct-17'            from dual union all
    select '31-Oct-17'            from dual union all
    select '18-Nov-18 12:11:10'   from dual union all
    select '31-Dec-12'            from dual union all
    select '31-Oct-17'            from dual union all
    select '31-Oct-17'            from dual union all
    select '18-Nov-18 12:11:10'   from dual union all
    select '31-Dec-12'            from dual union all
    select '31-Oct-17'            from dual union all
    select '31-Oct-17'            from dual union all
    select '20-Jul-18 19:20:33'   from dual union all
    select '31-Oct-18'            from dual union all
    select '31-Oct-18'            from dual union all
    select '11-Aug-19 21:52:56'   from dual union all
    select '31-Dec-12'            from dual
)
-- query
select to_char(to_date(substr(EXPIRY_DAT, 1, 9), 'dd-mon-yy'), 'yyyy')
from dateTable