Support ALTER TABLE DROP PRIMARY KEY

[ding-young]

Issue

resolves #591

[coveralls]

Pull Request Test Coverage Report for Build 3364446229

• 7 of 7 (100.0%) changed or added relevant lines in 3 files are covered.
• 411 unchanged lines in 5 files lost coverage.
• Overall coverage increased (+0.03%) to 85.974%

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Files with Coverage Reduction	New Missed Lines	%
src/ast/ddl.rs	1	82.29%
src/ast/value.rs	7	87.5%
src/ast/data_type.rs	13	88.62%
tests/sqlparser_common.rs	54	96.85%
src/parser.rs	336	83.72%

Totals
Change from base Build 3292360592:	0.03%
Covered Lines:	10923
Relevant Lines:	12705

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💛 - Coveralls

[AugustoFKL]

@ding-young ALTER TABLE tb DROP PRIMARY isn't allowed and, therefore, shouldn't be acceptable.

The let _ in the next line is permissive over this.

Also, I think this is a MySQL-specific statement. Could you please add the validation to accept only MySQL and GenericDialect, please?

[alamb]

@ding-young do you have time to implement @AugustoFKL 's suggestion? Otherwise this PR looks good to me

[ding-young]

@ding-young do you have time to implement @AugustoFKL 's suggestion? Otherwise this PR looks good to me

Yes, I'll work on it soon.

[ding-young]

@AugustoFKL Thank you for feedback. I changed the code 😃

[AugustoFKL]

LGTM

@alamb ready for approval :)

[alamb]

Thanks @ding-young and @AugustoFKL !