更改mysql字段的编码类型为utf8mb4

ALTER TABLE tb_case MODIFY COLUMN content VARCHAR(512) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE tb_case MODIFY COLUMN LAST_CHAT_CONTENT VARCHAR(512) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE tb_chat MODIFY COLUMN content VARCHAR(512) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE ofoffline MODIFY COLUMN stanza TEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;