#
# show the global and session values;
#
select @@global.datetime_format;
--error ER_INCORRECT_GLOBAL_LOCAL_VAR
select @@session.datetime_format;
show global variables like 'datetime_format';
show session variables like 'datetime_format';
select * from information_schema.global_variables where variable_name='datetime_format';
select * from information_schema.session_variables where variable_name='datetime_format';

#
# show that it's read-only
#
--error ER_INCORRECT_GLOBAL_LOCAL_VAR
set global datetime_format="foo";
--error ER_INCORRECT_GLOBAL_LOCAL_VAR
set session datetime_format="foo";