fix alter table convert to charset default syntax

[sim41]

What problem does this PR solve?

issue:#498

MySQL Syntax:

alter_specification:
      table_options
    | [DEFAULT] CHARACTER SET [=] charset_name [COLLATE [=] collation_name]
    | CONVERT TO CHARACTER SET charset_name [COLLATE collation_name]

It used to parse the CONVERT TO CHARACTER SET chaset_name into [DEFAULT] CHARACTER SET syntax. I think we just want to change only the default character set for a table.
(https://dev.mysql.com/doc/refman/8.0/en/alter-table.html).
So I reserved it.

Bad SQL Case:

ALTER TABLE d_n.t_n CONVERT TO CHARSET DEFAULT
ALTER TABLE d_n.t_n CONVERT TO CHAR SET DEFAULT
ALTER TABLE d_n.t_n CONVERT TO CHARACTER SET DEFAULT
ALTER TABLE d_n.t_n CONVERT TO CHARACTER SET DEFAULT COLLATE utf8_bin

Check List

Tests

• Unit test

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All committers have signed the CLA.

[codecov]

Codecov Report

Merging #566 into master will not change coverage.
The diff coverage is 70%.

@@          Coverage Diff           @@
##           master    #566   +/-   ##
======================================
  Coverage      80%     80%           
======================================
  Files          32      32           
  Lines       12681   12696   +15     
======================================
+ Hits        10145   10157   +12     
- Misses       1934    1935    +1     
- Partials      602     604    +2

Impacted Files	Coverage Δ
parser.go	92.94% <ø> (+0.01%)	⬆️
ast/ddl.go	80.17% <70%> (-0.09%)	⬇️

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[zier-one]

@tiancaiamao PTAL

[zier-one]

LGTM

[tiancaiamao]

PTAL @winkyao

[bb7133]

It used to parse the CONVERT TO CHARACTER SET chaset_name into [DEFAULT] CHARACTER SET syntax. I think we just want to change only the default character set for a table...

So I reserved it.

I think we need to support all syntaxes rather than CONVERT TO CHARSET DEFAULT. Please update the codes here for specified character name.

BTW, the current implementation of ALTER TABLE ... DEFAULT CHARSET in TiDB is wrong, I think we need to fix it later.

[sim41]

sry, kind of busy these days. I will try to fix the TiDB to support the parser later. And the CONVERT TO CHARSET DEFAULT is another commits that has been merged. I will fix all the syntaxes as what MySQL do.

[bb7133]

hi @sim41 , thanks for the contribution, please check my comments.

[zier-one]

hi @sim41 , could you check the comments and fix conflicts please?

[tiancaiamao]

Ping @sim41

[sim41]

@tiancaiamao @tangenta sry, kind of busy these days. I will fix it this weekends.

[zier-one]

@bb7133 PTAL

[bb7133]

LGTM

[bb7133]

hi @sim41 please resolve the conflict and we can approve your PR, thanks for the contribution!