SQL> use test;

SQL> CREATE TABLE `mail` (

  `subject` varchar(255) DEFAULT NULL,

  `body` varchar(255) DEFAULT NULL,

  `attachement` longblob DEFAULT NULL,

  `metadata` longtext CHARACTER SET utf8mb4 COLLATE utf8mb4_bin DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=latin1

;

SQL> ALTER TABLE mail MODIFY COLUMN `metadata` LONGTEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_bin COMPRESSED DEFAULT NULL;

SQL> SHOW CREATE TABLE mail\G

*************************** 1. row ***************************

       Table: mail

Create Table: CREATE TABLE `mail` (

  `subject` varchar(255) DEFAULT NULL,

  `body` varchar(255) DEFAULT NULL,

  `attachement` longblob DEFAULT NULL,

  `metadata` longtext /*!100301 COMPRESSED*/ CHARACTER SET utf8mb4 COLLATE utf8mb4_bin DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=latin1

# mysqldump -u root test > test_dump.sql

# mysql -uroot test < test_dump.sql 

ERROR 1064 (42000) at line 25: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'CHARACTER SET utf8mb4 COLLATE utf8mb4_bin DEFAULT NULL

) ENGINE=InnoDB DEFAULT C' at line 5

CREATE TABLE `mail2` (

  `subject` varchar(255) DEFAULT NULL,

  `body` varchar(255) DEFAULT NULL,

  `attachement` longblob DEFAULT NULL,

  `metadata` longtext /*!100301 COMPRESSED*/ CHARACTER SET utf8mb4 COLLATE utf8mb4_bin DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=latin1;

CREATE TABLE `mail3` (

  `subject` varchar(255) DEFAULT NULL,

  `body` varchar(255) DEFAULT NULL,

  `attachement` longblob DEFAULT NULL,

  `metadata` longtext CHARACTER SET utf8mb4 COLLATE utf8mb4_bin /*!100301 COMPRESSED*/ DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=latin1;