--disable_warnings
DROP TABLE IF EXISTS t1;
--enable_warnings

CREATE TABLE t1 (c1 ENUM('a', '', 'b'), c2 INT DEFAULT 1); 

INSERT INTO t1 (c1) VALUES (0), ('a'), (''), ('b');

SELECT c1 + 0, c1 FROM t1 GROUP BY c1;
SELECT c1 + 0, c1, SUM(c2) FROM t1 GROUP BY c1;

DROP TABLE t1;