union result is not same as mysql

[H-ZeX]

Bug Report

1. What did you do?
   If possible, provide a recipe for reproducing the error.

create table t(a int);
create table s(a int unsigned);

insert into t value(-1);
insert into s value(1);

2. What did you expect to see?

select * from t union all select * from s;
+------+
| a    |
+------+
|   -1 |
|    1 |
+------+

select * from s union all select * from t;
+------+
| a    |
+------+
|   -1 |
|    1 |
+------+

3. What did you see instead?

select * from t union all select * from s;
+------+
| a    |
+------+
|   -1 |
|    1 |
+------+

select * from s union all select * from t;
+------+
| a    |
+------+
|    0 |
|    1 |
+------+

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

Release Version: v4.0.0-alpha-15-gd2b538718
Git Commit Hash: d2b5387183a591847f952aa4e3a3c4597c34dcba
Git Branch: HEAD
UTC Build Time: 2019-08-02 08:50:35
GoVersion: go version go1.12.6 linux/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false

mysqld  Ver 8.0.13 for linux-glibc2.12 on x86_64 (MySQL Community Server - GPL)

5. relate issue
   union result is not differ from mysql #7075

[zz-jason]

@SunRunAway PTAL

[SunRunAway]

Hi, @H-ZeX
could you provide the version of your MySQL server?

[H-ZeX]

Hi, @H-ZeX
could you provide the version of your MySQL server?

mysqld  Ver 8.0.13 for linux-glibc2.12 on x86_64 (MySQL Community Server - GPL)

@SunRunAway

[SunRunAway]

TiDB aims to compatible with MySQL5.7 so it seems to be fixed in #7075

[SunRunAway]

Hi, @H-ZeX
I'm closing this issue now, feel free to reopen it if I'm wrong.