Mysql Why select count distinct return zero

Question

I would like to know why select count distinct return zero result. I have also checked various answers at here but it's not answer for my case. MySQL version 5.6 Link to check http://sqlfiddle.com/#!9/276302/3/0

Sample schema:

CREATE TABLE IF NOT EXISTS `employees` (
  `id` int(6) unsigned NOT NULL,
  `name` varchar(3) NOT NULL,
  `salary` varchar(200) NOT NULL,
  PRIMARY KEY (`id`)
) DEFAULT CHARSET=utf8;
INSERT INTO `employees` (`id`, `name`, `salary`) VALUES
  ('1', 'a', 6),
  ('2', 'b', 5),
  ('3', 'c', 5),
  ('4', 'd', 4);

Query:

SELECT COUNT(DISTINCT(salary))
FROM employees;

Guys, do you have any idea ?

Answer 1

In my opinion it is a bug of this specific version of MySQL 5.6.

But it will work for MariaDB or MySQL 5.7

The other answers suggest that it is the problem with datatype.But if we remove DISTINCT:

SELECT COUNT(salary)
FROM employees;
-- 4

DBFiddle Demo MySQL 5.6

Answer 2

select count(ds) from (SELECT distinct(salary) as ds
FROM employees) as s;