Formatting date when using CAST - sql

Question

I would like to use CAST to convert a DATE type to a VARCHAR2 type.
DBUSER >SELECT CAST(CURRENT_DATE AS VARCHAR2(20)) THE_DATE from DUAL;
THE_DATE
--------------------
09-AUG-17
However, I need the VARCHAR2 result to be formatted as 'YYYYMM'. I know that I can achieve this effect by changing the session date format, but I would rather not do that.
DBUSER >ALTER SESSION SET NLS_DATE_FORMAT = 'YYYYMM';
Session altered.
DBUSER >SELECT CAST(CURRENT_DATE AS VARCHAR2(20)) THE_DATE from DUAL;
THE_DATE
--------------------
201708
I would like to avoid using Oracle's proprietary TO_CHAR() function. Does anyone have a suggestion on how to do that?

Answer 1

I am trying to standardize on ANSI SQL to the degree possible and avoid proprietary vendor nonstandard implementations.
There is no function specified in the ANSI SQL92 standard which formats DATETIME datatypes as a string.
The simplest solution is to use the functions Oracle provides for that purpose:
SELECT TO_CHAR( yourdate, 'YYYYMM' ) FROM yourtable;
However, you can get the year and month components using the EXTRACT function (which is in the ANSI standard):
SELECT EXTRACT( YEAR FROM yourdate ),
EXTRACT( MONTH FROM yourdate )
FROM yourtable;
Then you need to convert the numbers to a string and concatenate the strings:
SELECT TO_CHAR( EXTRACT( YEAR FROM yourdate ) )
|| TO_CHAR( EXTRACT( MONTH FROM yourdate ) )
FROM yourtable
but you were trying to avoid TO_CHAR so you could do:
SELECT CAST( EXTRACT( YEAR FROM yourdate ) AS VARCHAR2(4) )
|| CAST( EXTRACT( MONTH FROM yourdate ) AS VARCHAR2(2) )
FROM yourtable
or, using an implicit cast
SELECT EXTRACT( YEAR FROM yourdate )
|| EXTRACT( MONTH FROM yourdate )
FROM yourtable
However, if the year is not 4-digits or the month is not 2-digits then you need to pad the values; again, the simple solution is TO_CHAR:
SELECT TO_CHAR( EXTRACT( YEAR FROM yourdate ), 'FM0000' )
|| TO_CHAR( EXTRACT( MONTH FROM yourdate ), 'FM00' )
FROM yourtable
or LPAD:
SELECT LPAD( EXTRACT( YEAR FROM yourdate ), 4, '0' )
|| LPAD( EXTRACT( MONTH FROM yourdate ), 4, '0' )
FROM yourtable
But neither of those are in the ANSI standard so:
SELECT CASE
WHEN EXTRACT( YEAR FROM yourdate ) < 10 THEN '000'
WHEN EXTRACT( YEAR FROM yourdate ) < 100 THEN '00'
WHEN EXTRACT( YEAR FROM yourdate ) < 1000 THEN '0'
ELSE NULL
END
|| EXTRACT( YEAR FROM yourdate )
|| CASE
WHEN EXTRACT( MONTH FROM yourdate ) < 10 THEN '0'
END
|| EXTRACT( MONTH FROM yourdate )
FROM yourtable;
And we've managed to transform a single Oracle function into a behemoth of an ANSI compatible expression.
But, Oracle's DATE datatype does not comply to the ANSI standard (it is a concatenation of the ANSI DATE and TIME datatypes) so I'll ask whether it is worth it - especially if you then consider displaying the time component of a date (which EXTRACT will not extract unless you first use CAST to convert the DATE to a TIMESTAMP).
SELECT TO_CHAR( yourdate, 'YYYYMMDDHH24MISS' ) FROM yourtable
or
SELECT CASE
WHEN EXTRACT( YEAR FROM yourdate ) < 10 THEN '000'
WHEN EXTRACT( YEAR FROM yourdate ) < 100 THEN '00'
WHEN EXTRACT( YEAR FROM yourdate ) < 1000 THEN '0'
ELSE NULL
END
|| EXTRACT( YEAR FROM yourdate )
|| CASE
WHEN EXTRACT( MONTH FROM yourdate ) < 10 THEN '0'
END
|| EXTRACT( MONTH FROM yourdate )
|| CASE
WHEN EXTRACT( DAY FROM yourdate ) < 10 THEN '0'
END
|| EXTRACT( DAY FROM yourdate )
|| CASE
WHEN EXTRACT( HOUR FROM CAST( yourdate AS TIMESTAMP ) ) < 10 THEN '0'
END
|| EXTRACT( HOUR FROM CAST( yourdate AS TIMESTAMP ) )
|| CASE
WHEN EXTRACT( MINUTE FROM CAST( yourdate AS TIMESTAMP ) ) < 10 THEN '0'
END
|| EXTRACT( MINUTE FROM CAST( yourdate AS TIMESTAMP ) )
|| CASE
WHEN EXTRACT( SECOND FROM CAST( yourdate AS TIMESTAMP ) ) < 10 THEN '0'
END
|| EXTRACT( SECOND FROM CAST( yourdate AS TIMESTAMP ) )
FROM yourtable;
[TL/DR] Just use TO_CHAR

Answer 2

This may be help you:
SELECT extract(year from CURRENT_DATE) || case when extract(month from CURRENT_DATE) <10 THEN '0' || extract(month from CURRENT_DATE) END THE_DATE from DUAL;