• BINARY

  SET NAMES utf8;
  DROP TABLE IF EXISTS t1;
  CREATE TABLE t1 (a BINARY(30));
  INSERT INTO t1 VALUES ('1äÖüß@µ*$');
  SELECT CAST(a AS DECIMAL) FROM t1;
  SHOW WARNINGS;
  

  +---------+------+---------------------------------------------------------------------------------------------------------------------------------------------------+
  | Level   | Code | Message                                                                                                                                           |
  +---------+------+---------------------------------------------------------------------------------------------------------------------------------------------------+
  | Warning | 1292 | Truncated incorrect DECIMAL value: '1\xC3\xA4\xC3\x96\xC3\xBC\xC3\x9F@\xC2\xB5*$\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00' |
  +---------+------+---------------------------------------------------------------------------------------------------------------------------------------------------+
  

• VARBINARY

  SET NAMES utf8;
  DROP TABLE IF EXISTS t1;
  CREATE TABLE t1 (a VARBINARY(30));
  INSERT INTO t1 VALUES ('1äÖüß@µ*$');
  SELECT CAST(a AS DECIMAL) FROM t1;
  SHOW WARNINGS;
  

  +---------+------+-----------------------------------------------------+
  | Level   | Code | Message                                             |
  +---------+------+-----------------------------------------------------+
  | Warning | 1292 | Truncated incorrect DECIMAL value: '1äÖüß@µ*$'      |
  +---------+------+-----------------------------------------------------+
  

• BLOB

  SET NAMES utf8;
  DROP TABLE IF EXISTS t1;
  CREATE TABLE t1 (a BLOB);
  INSERT INTO t1 VALUES ('1äÖüß@µ*$');
  SELECT CAST(a AS DECIMAL) FROM t1;
  SHOW WARNINGS;
  

  Empty set (0.00 sec)
  

BINARY looks the most correct.