TO_DATE function in ORACLE

Question

I was trying the TO_DATE function. Specifically, I noted that the following queries

1. SELECT TO_CHAR(TO_DATE(\'01-01-2015\',\'DD-MM-YYYY\'),\'DD-MO         


        

Answer 1

Dates do not have a format - they are represented by 7- or 8-bytes.

SELECT DUMP( SYSDATE ) FROM DUAL;

Might output:

Typ=13 Len=8: 220,7,11,26,16,41,9,0

This format is very useful for computers to compare dates but not so useful to people; so, when the SQL client (SQL/plus, SQL Developers, TOAD, etc) displays a date it does not display the the bytes but displays it as a string.

It does this by making an implicit call to TO_CHAR() (or some other internal method of stringifying dates) and uses a default format mask to perform this conversion.

SQL/Plus and SQL Developer will use the user's session parameter NLS_DATE_FORMAT to perform this conversion - see this answer regarding this.

So your second query is implicitly being converted to do something approaching this (but, almost certainly, more efficiently):

SELECT TO_CHAR(
         TO_DATE('01-01-2015','DD-MM-YYYY'),
         ( SELECT VALUE FROM NLS_SESSION_PARAMETERS WHERE PARAMETER = 'NLS_DATE_FORMAT' )
       )
FROM   DUAL