DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (c1 ENUM('a', '', 'b'), c2 INT DEFAULT 1);
INSERT INTO t1 (c1) VALUES (0), ('a'), (''), ('b');
Warnings:
Warning	1265	Data truncated for column 'c1' at row 1
SELECT c1 + 0, c1 FROM t1 GROUP BY c1;
c1 + 0	c1
0	
1	a
2	
3	b
SELECT c1 + 0, c1, SUM(c2) FROM t1 GROUP BY c1;
c1 + 0	c1	SUM(c2)
1	a	1
0		2
3	b	1
DROP TABLE t1;