Auto_increment is not compatible with MySQL

[ChenPeng2013]

Bug Report

Please answer these questions before submitting your issue. Thanks!

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

create table t2(a int auto_increment, b int key);

2. What did you expect to see?
   mysql 8.0.17
   

3. What did you see instead?
   tidb
   

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

Release Version: v2.1.17-41-g8e80122af
Git Commit Hash: 8e80122af807c402cc7ca26adde4d14e9fa7f9b9
Git Branch: HEAD
UTC Build Time: 2019-10-29 02:19:18
GoVersion: go version go1.13 linux/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false

[SunRunAway]

@bb7133 PTAL

[zimulala]

In MySQL 5.7.20, this SQL also reports errors.

[Rustin170506]

I am working on it.

[zimulala]

@Rustin-Liu
Looking forward to your PR.