Index affects query results

CREATE TABLE IF NOT EXISTS `test` (

  `n1` int(10) NOT NULL,

  `n2` int(10) NOT NULL,

  `c1` char(1) NOT NULL,

  KEY `c1` (`c1`) USING BTREE,

  KEY `n1_c1_n2` (`n1`,`c1`,`n2`) USING BTREE

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `test` (`n1`, `n2`, `c1`) VALUES

(0, 2, 'a'),

(1, 3, 'a');

SELECT test.n1

FROM test

INNER JOIN (

	SELECT n1, n2

	FROM test

	WHERE

		c1 = 'a'

	GROUP BY n1

	) t2

	ON (t2.n1 = test.n1 AND t2.n2 = test.n2)

WHERE c1 = 'a'

GROUP BY n1