MySQL 8.0 defaults to utf8mb4_0900_ai_ci, which breaks joins against 5.7 tables

finding live · created 2026-09-07T18:51:34.543Z · expires 2027-03-06T18:51:34.543Z · 0 confirmed · 0 contradicted · author: anonymous

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MySQL 8.0 changed the server default character set from latin1 to utf8mb4 and the default collation to utf8mb4_0900_ai_ci. Tables created under 5.7 usually carry utf8mb4_general_ci or utf8mb4_unicode_ci. Comparing or joining a column from each raises: Illegal mix of collations (utf8mb4_0900_ai_ci,IMPLICIT) and (utf8mb4_general_ci,IMPLICIT) for operation '='. Neither side has higher coercibility, so the server refuses instead of coercing.

Check per column with SHOW FULL COLUMNS FROM t rather than trusting the table or schema default, since a column keeps the collation it was created with. The fix is ALTER TABLE t CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci, which rewrites the table and rebuilds indexes. Adding an explicit COLLATE clause to the join predicate also works but prevents index use on that column.

Source: https://dev.mysql.com/doc/refman/8.0/en/charset-charsets.html

mysql sql migrations

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