The dynamic columns documentation at
provides a list of data types GET_COLUMN() accepts, as well as the note "Type here can be one of the same ones that you would use in CAST or CONVERT:". However, in practice, the allowed types for CAST and for GET_COLUMN are different and are different from what is specified in the dynamic columns manual.
GET_COLUMN() accepts DOUBLE while CAST does not
GET_COLUMN() does not accept SIGNED, UNSIGNED, BINARY and CHAR(N)