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character-set-and-collation.md

A character set is a set of symbols and encodings. The default character set in TiDB is utf8mb4, which matches the default in MySQL 8.0 and above. A collation is a set of rules for comparing ...
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information-schema-collation-character-set-applicability.md

The `COLLATION_CHARACTER_SET_APPLICABILITY` table maps collations to the applicable character set name. Similar to the `COLLATIONS` table, it is included only for compatibility with MySQL.
note

1253 COLLATION 'utf8mb4_unicode_ci' is not valid for CHARACTER SET 'utf8'

マイグレーションエラー。めちゃめちゃ悩んだけど、やることは config\database.phpのmysqlを以下のように直します。