(WIP) Adds support for generated columns in mysql (in create statements)

[jspaezp]

Brief summary of the change made

This PR attempts to add support for generated columns in the mysql dialect.
Related: #4960

Are there any other side effects of this change that we should be aware of?

Not that I know ...

Pull Request checklist

• Please confirm you have completed any of the necessary steps below.

  • Added test file, will generate yml when I finish the implementation.

• Included test cases to demonstrate any code changes, which may be one or more of the following:

  • .yml rule test cases in test/fixtures/rules/std_rule_cases.
  • .sql/.yml parser test cases in test/fixtures/dialects (note YML files can be auto generated with tox -e generate-fixture-yml).
  • Full autofix test cases in test/fixtures/linter/autofix.
  • Other.

• Added appropriate documentation for the change.

• Created GitHub issues for any relevant followup/future enhancements if appropriate.

[jspaezp]

Hello there! I am not 100% sure how to finish the implementation to get this past the finish line ...

What I have:
If I am understanding correctly... I want to implement a GeneratedColumnDefinitionSegment (in the PR)

BUT I am failing to understand how the copy -> insert/replace/add mechanism works. IF I AM UNDERSTANDING THIS CORRECTLY ... the mysql dialect uses ColumnDefinitionSegment (

sqlfluff/src/sqlfluff/dialects/dialect_mysql.py

Line 319 in 1d4fd8d

class ColumnDefinitionSegment(BaseSegment):

) because it is used in the ansi dialect definition of CreateTableStatementSegment (https://github.com/jspaezp/sqlfluff/blob/1d4fd8d288e9d19aa544d3d92885b30788234470/src/sqlfluff/dialects/dialect_ansi.py#L3188) ... if I wanted to 'insert' into that definition

                    Delimited(
                        OneOf(
                            Ref("TableConstraintSegment"),
                            Ref("ColumnDefinitionSegment"),
                            # <------- HERE add `Ref("GeneratedColumnDefinitionSegment")`
                        ),
                    )

that could be done in ... here (mysql re-definition of CreateTableStatementSegment)

sqlfluff/src/sqlfluff/dialects/dialect_mysql.py

Lines 377 to 394 in 1d4fd8d

	class CreateTableStatementSegment(ansi.CreateTableStatementSegment):
	"""Create table segment.
	https://dev.mysql.com/doc/refman/8.0/en/create-table.html
	"""
	match_grammar = ansi.CreateTableStatementSegment.match_grammar.copy(
	insert=[
	AnyNumberOf(
	Sequence(
	Ref.keyword("DEFAULT", optional=True),
	Ref("ParameterNameSegment"),
	Ref("EqualsSegment", optional=True),
	OneOf(Ref("LiteralGrammar"), Ref("ParameterNameSegment")),
	),
	),
	],
	)

... but I do not 100% understand what the ansi.CreateTableStatementSegment.match_grammar.copy(insert= [...]) pattern does.
Any help would be appreciated! (edited)

[greg-finley]

Thanks for the PR, but closing in favor of #5939