vague alias usage in GROUP BY clause

DROP TEMPORARY TABLE IF EXISTS `test_set`;

CREATE TEMPORARY TABLE `test_set`

(

    `id`     INT NOT NULL auto_increment PRIMARY KEY,

    `tag`    VARCHAR(255),

    `tag2`  VARCHAR(255)

);

INSERT INTO test_set (tag, tag2)

VALUES ('hello', 'a'),

       ('hello', 'b'),

       ('world', 'c');

SELECT COUNT(*), tag2 as tag

FROM test_set

GROUP BY tag;

SELECT COUNT(*), tag2 as tag3

FROM test_set

GROUP BY tag3;