Aggregating enum zero value gets different results from mysql

[wshwsh12]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

SET sql_mode = 'NO_ENGINE_SUBSTITUTION';
drop table t1;
CREATE TABLE t1 (c1 ENUM('a', '', 'b'));
INSERT INTO t1 (c1) VALUES ('b');
INSERT INTO t1 (c1) VALUES ('');
INSERT INTO t1 (c1) VALUES (0);
INSERT INTO t1 (c1) VALUES ('');
SELECT c1 + 0, COUNT(c1) FROM t1 GROUP BY c1 order by c1;

This situation can only occur with the following conditions:

1. Enum type has '' name field.
2. Illegal value/zero value is inserted in the data.
   So I think this problem is not very serious..

2. What did you expect to see? (Required)

MySQL [test]> SELECT c1 + 0, COUNT(c1) FROM t1 GROUP BY c1 order by c1;
+--------+-----------+
| c1 + 0 | COUNT(c1) |
+--------+-----------+
|      0 |         1 |
|      2 |         2 |
|      3 |         1 |
+--------+-----------+
3 rows in set (0.000 sec)

3. What did you see instead (Required)

[tidb]> SELECT c1 + 0, COUNT(c1) FROM t1 GROUP BY c1 order by c1;
+--------+-----------+
| c1 + 0 | COUNT(c1) |
+--------+-----------+
|      2 |         3 |
|      3 |         1 |
+--------+-----------+
2 rows in set (0.001 sec)

// Different insert order will get different result. If I insert values(0) first, the result will be following:

[tidb]> SELECT c1 + 0, COUNT(c1) FROM t1 GROUP BY c1 order by c1;
+--------+-----------+
| c1 + 0 | COUNT(c1) |
+--------+-----------+
|      0 |         3 |
|      3 |         1 |
+--------+-----------+
2 rows in set (0.001 sec)

4. What is your TiDB version? (Required)

master, v5.0 v5.1, v4.0