MySQL 8.0

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CREATE TABLE test (
  txt1 VARCHAR(10) CHARACTER SET UTF8 COLLATE utf8_unicode_ci,
  txt2 VARCHAR(10) CHARACTER SET UTF8 COLLATE utf8_general_ci
  );
INSERT INTO test VALUES ('Проба', 'ба'), ('пера', 'ра');

Records: 2  Duplicates: 0  Warnings: 0

SELECT * FROM test WHERE txt1 = CONCAT('Про', txt2);

Illegal mix of collations (utf8mb3_unicode_ci,IMPLICIT) and (utf8mb3_general_ci,IMPLICIT) for operation '='

SELECT * FROM test WHERE txt1 = CONCAT('Про', txt2) COLLATE utf8mb3_unicode_ci;

txt1
txt2

Проба
ба