• 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.