The use of mysql unique key in query and related issues

CREATE TABLE `employees` (
 `emp_no` int(11) NOT NULL,
 `birth_date` date NOT NULL,
 `first_name` varchar(14) NOT NULL,
 `last_name` varchar(16) NOT NULL,
 `gender` char(1) NOT NULL,
 `hire_date` date NOT NULL,
 PRIMARY KEY (`emp_no`),
 UNIQUE KEY `idxunique_first_name_gender` (`first_name`,`gender`),
 UNIQUE KEY `uniq_idx_firstname` (`first_name`,`gender`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

-- Add a unique index alter table employees add constraint idxunique_first_name_gender unique(first_name, gender);

-- Include all fields in the index, and the order is consistent with the index select * from employees where first_name='Chirstian' and gender='M';
-- key column value: idxunique_first_name_gender
-- Include all fields in the index, but the order and index are inconsistent select * from employees where gender='M' and first_name='Chirstian';
-- key column value: idxunique_first_name_gender
-- Include index on first field select * from employees where first_name='Chirstian';
-- key column value: idxunique_first_name_gender
-- Include non-first fields in the index select * from employees where gender='M';
-- key column value: NULL

get user_id=10000 from xxx_tab;
if exist 
{
  return ok
}
ret = insert user_id=10000
if ret ok 
{
  return ok
}
return not_ok