sql-statements: document ALTER TABLE COMPRESSION='NONE' behavior

[takaidohigasi]

What is changed, added or deleted? (Required)

Document the ALTER TABLE ... COMPRESSION behavior introduced by pingcap/tidb#62305:

• ALTER TABLE ... COMPRESSION only accepts 'NONE' (case-insensitive) and it is a no-op (metadata-only; TiDB does not compress or uncompress any table data). This allows reverting the COMPRESSION attribute for tables migrated from MySQL.
• Other values such as 'ZLIB' and 'LZ4' return the error This type of ALTER TABLE is currently unsupported (error code 8200).
• If an unsupported COMPRESSION value is specified together with other options in one statement, none of the options take effect.

Added one item to the MySQL compatibility section of sql-statements/sql-statement-alter-table.md.

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What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): ddl: enable ALTER TABLE COMPRESSION=NONE (no ops) tidb#62305

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

• Delete files
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• Might cause conflicts after applied to another branch

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[gemini-code-assist]

Code Review

This pull request updates the ALTER TABLE documentation to clarify that the COMPRESSION option only accepts NONE as a no-op for compatibility, while other values like ZLIB or LZ4 will return an error. The review feedback suggests formatting SQL values with backticks, using active voice, and addressing the user in the second person to align with the repository style guide.

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[qiancai]

@D3Hunter Please help with the tech review of this PR. Thanks.

[qiancai]

The parser grammar requires COMPRESSION to be followed by stringLit, so bare NONE, ZLIB, and LZ4 do not reflect executable syntax. Keeping the quotes inside the code span also makes the MySQL-migration example immediately usable.

Suggested change

- `ALTER TABLE ... COMPRESSION` only accepts `NONE` (case-insensitive) and is a no-op. TiDB accepts this statement only for compatibility (for example, for tables migrated from MySQL that carry a `COMPRESSION` attribute) and does not compress or uncompress any table data. Specifying other values, such as `ZLIB` or `LZ4`, returns the error `This type of ALTER TABLE is currently unsupported` (error code 8200). If you specify an unsupported `COMPRESSION` value together with other options in a single statement, none of the options take effect.

- `ALTER TABLE ... COMPRESSION` accepts only the string literal `'NONE'` (case-insensitive) and is a no-op. TiDB accepts this statement only for compatibility, for example, for tables migrated from MySQL that carry a `COMPRESSION` attribute. It does not compress or uncompress table data. Other values, such as `'ZLIB'` or `'LZ4'`, return error code 8200 (`This type of ALTER TABLE is currently unsupported`). If you specify an unsupported `COMPRESSION` value together with other options in a single statement, none of the options take effect.