Modifying a query to work without requiring NLS_DATE_FORMAT to be set

Question

I the following query:

SELECT
    trunc(estimatedenddate,'hh') AS reg_date,
    COUNT(*)
FROM
    (
        SELECT
            attr_value,
            TO_DATE( (DATE '1970-01-01' + (1 / 24 / 60 / 60 / 1000) * attr_value),'yyyy-mm-dd HH24:mi:ss') AS estimatedenddate
        FROM
            attr
        WHERE
            attr_name = 'createTimestamp'
    )
WHERE
    estimatedenddate > TO_DATE('01/JUN/2018','dd/mon/yyyy')
GROUP BY
    trunc(estimatedenddate,'hh')
ORDER BY
    reg_date DESC;

It works when I set the NLS_DATE_FORMAT . See dbfiddle .

However if I do not set the NLS_DATE_FORMAT , the query doesn't produce any results. See dbfiddle

How can I modify this query such that it works without setting the NLS_DATE_FORMAT ?

Answer 1

TO_DATE( date_string, format_model ) takes a string for the first argument but you are passing a DATE data type which Oracle will try to be helpful and implicitly convert to the expected string data type using its default format model; so your inner query is effectively:

SELECT attr_value,
       TO_DATE(
         TO_CHAR(
           DATE '1970-01-01' + (1 / 24 / 60 / 60 / 1000) * attr_value,
           (SELECT value FROM NLS_SESSION_PARAMETERS WHERE parameter = 'NLS_DATE_FORMAT')
         ),
         'yyyy-mm-dd HH24:mi:ss'
       ) AS estimatedenddate
FROM   attr
WHERE  attr_name = 'createTimestamp'

Instead you can just remove the TO_DATE() function:

SELECT trunc(estimatedenddate,'hh') AS reg_date,
       COUNT(*)
FROM   (
  SELECT DATE '1970-01-01' + (1 / 24 / 60 / 60 / 1000) * attr_value AS estimatedenddate
  FROM   attr
  WHERE  attr_name = 'createTimestamp'
)
WHERE    estimatedenddate > DATE '2018-06-01'
GROUP BY trunc(estimatedenddate,'hh')
ORDER BY reg_date DESC;