float type incompatible behavior from mysql

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Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t2(a float, b float, c float as (a % 10), key (c));
insert into t2 (a) values (-100), (20);
select c from t2 union select c from t2;
select c from t2;
select * from t2;

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

mysql>  select c from t2 union select c from t2;
+------+
| c    |
+------+
|   -0 |
|    0 |
+------+
2 rows in set (0.03 sec)

mysql> select c from t2;
+------+
| c    |
+------+
|   -0 |
|    0 |
+------+
2 rows in set (0.04 sec)

mysql> select * from t2;
+------+------+------+
| a    | b    | c    |
+------+------+------+
| -100 | NULL |   -0 |
|   20 | NULL |    0 |
+------+------+------+
2 rows in set (0.04 sec)

3. What did you see instead (Required)

mysql> create table t2(a float, b float, c float as (a % 10), key (c));
Query OK, 0 rows affected (0.18 sec)

mysql> insert into t2 (a) values (-100), (20);
Query OK, 2 rows affected (0.09 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select c from t2 union select c from t2;
+---+
| c |
+---+
| 0 |
+---+
1 row in set (0.07 sec)

mysql>
mysql> select c from t2;
+------+
| c    |
+------+
|    0 |
|    0 |
+------+
2 rows in set (0.04 sec)

mysql> select * from t2;
+------+------+------+
| a    | b    | c    |
+------+------+------+
| -100 | NULL |   -0 |
|   20 | NULL |    0 |
+------+------+------+
2 rows in set (0.03 sec)

4. What is your TiDB version? (Required)

5.0.3
4.0.14