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 (#337 , e.g) and MySQL8.0 (#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:

• parser: fix tableAlias in DeleteStmt #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:

• Fix SECONDARY_LOAD/SECONDARY_UNLOAD syntax #444, solved in parser: support secondary_load and secondary_unload #496

• Fix CHECK syntax #497, solved in parser: support ALTER TABLE ALTER CHECK, ALTER TABLE DROP CHECK syntax #505

• parser: fix tableAlias in DeleteStmt #521 , tableAlias is supported only in MySQL 8.0

The MariaDB compatibility issues:

• *: add IF EXISTS and IF NOT EXISTS supported for MariaDB #337 and improved in parser: fix IF NOT EXISTS support for MariaDB ADD PARTITION syntax #455

[winkyao]

Agree, Could you write a proposal for this?