Keep track of compatibility issues for various of MySQL version

[spongedu]

Recently I'm working on improving TiDB parser's compatibility( see Improve parser compatibility). As we all know, parser's behavior is mostly base on MySQL5.7, but as time goes by there are some MariaDB (https://github.com/pingcap/parser/pull/337 , e.g) and MySQL8.0 (https://github.com/pingcap/parser/pull/496 e.g) feature join in. Maybe we could add some control here so that the parser could stick to one version at a time.

I think it's a good start to keep track of all the issues that is for MySQL 8.0, MariaDB or anything not MySQL 5.7. Maybe we should add tags for these issue, as well.

The MySQL 5.7 compatibility issues:

• #521 , the .identifier is valid as TableName only in 5.7 and lower, and is deprecated in MySQL 8.0 The MySQL 8.0 compatibility issues:

• #444, solved in #496

• #497, solved in #505

• https://github.com/pingcap/parser/pull/521 , tableAlias is supported only in MySQL 8.0

The MariaDB compatibility issues:

• #337 and improved in #455

[winkyao]

Agree, Could you write a proposal for this?