Question

using date_format as specified in the doc from mysql

select date_format('20-May-2013', '%d-%b-%Y');

returns null, why?

Answer 1

You are trying to parse a date using DATE_TIME(), but that function is meant to do the inverse – take a known formatted date and display it using a custom format. What you probably want is the STR_TO_DATE() function:

select str_to_date('20-May-2013', '%d-%b-%Y');