create user test_user@localhost;
create role test_role;
grant select on *.* to test_role;
grant test_role to test_user@localhost;
show grants;
Grants for test_user@localhost
GRANT test_role TO 'test_user'@'localhost'
GRANT USAGE ON *.* TO 'test_user'@'localhost'
set default role test_role;
select user, host, default_role from mysql.user;
ERROR 42000: SELECT command denied to user 'test_user'@'localhost' for table 'user'
select user, host, default_role from mysql.user where user='test_user';
user	host	default_role
test_user	localhost	test_role
show grants;
Grants for test_user@localhost
GRANT test_role TO 'test_user'@'localhost'
GRANT USAGE ON *.* TO 'test_user'@'localhost'
GRANT SELECT ON *.* TO 'test_role'
select user, host, default_role from mysql.user where user='test_user';
user	host	default_role
test_user	localhost	test_role
set default role NONE;
select user, host, default_role from mysql.user where user='test_user';
user	host	default_role
test_user	localhost	
set default role invalid_role;
ERROR OP000: Invalid role specification `invalid_role`.
select user, host, default_role from mysql.user where user='test_user';
user	host	default_role
test_user	localhost	
select user, host, default_role from mysql.user;
ERROR 42000: SELECT command denied to user 'test_user'@'localhost' for table 'user'
drop role test_role;
drop user test_user@localhost;