The difference between the date format ' yyyy-mm-dd ' and ' YYYYMMDD ' in Oracle

For years and Days "Day" is a single-digit case, the processing is not the same, ' YYYYMMDD ' format is not a problem, and the format ' yyyy-mm-dd ' form is not, please see:

Sql> alter session Set Nls_date_format = ' Yyyy-mm-dd hh24:mi:ss ';

Session altered.

Sql> Select To_date (' 2007059 ', ' yyyy-mm-dd hh24:mi:ss ') from dual;
Select To_date (' 2007059 ', ' yyyy-mm-dd hh24:mi:ss ') from dual
 *
ERROR at line 1:
Ora-01861:literal does not match format string

Sql> Select To_date (' 2007059 ', ' YYYY-MM-DD ') from dual;
Select To_date (' 2007059 ', ' YYYY-MM-DD ') from dual
 *
ERROR at line 1:
Ora-01861:literal does not match format string

Sql> Select To_date (' 2007059 ', ' YYYYMMDD ') from dual;

To_date (' 2007059 ', '

The difference between the date format ' yyyy-mm-dd ' and ' YYYYMMDD ' in Oracle