select @@global.date_format;
@@global.date_format
%Y-%m-%d
Warnings:
Warning	1287	'@@date_format' is deprecated and will be removed in a future release.
select @@session.date_format;
ERROR HY000: Variable 'date_format' is a GLOBAL variable
show global variables like 'date_format';
Variable_name	Value
date_format	%Y-%m-%d
show session variables like 'date_format';
Variable_name	Value
date_format	%Y-%m-%d
select * from information_schema.global_variables where variable_name='date_format';
VARIABLE_NAME	VARIABLE_VALUE
DATE_FORMAT	%Y-%m-%d
select * from information_schema.session_variables where variable_name='date_format';
VARIABLE_NAME	VARIABLE_VALUE
DATE_FORMAT	%Y-%m-%d
set global date_format="foo";
ERROR HY000: Variable 'date_format' is a read only variable
set session date_format="foo";
ERROR HY000: Variable 'date_format' is a read only variable
