I suggest to use for testing a numeric field for joining on due to trick for char coding, space and some characters.

LAYER
  NAME "mysqltest"
  STATUS ON
  DATA "/var/www/map/dati/mysqljointest"
  TYPE POLYGON
  TEMPLATE void
  METADATA
    "DESCRIPTION"  "mysqljointtest"
    "RESULT_FIELDS" "ED,DESCRIZIONE"
    "RESULT_HEADERS" "Descrizione,MySQL"
    "RESULT_JOIN" "mysql://<user>:<password>@localhost/<database>||<table>@ED@0@DESCRIZIONE||ED||1"
  END
  CLASS
    NAME "mysqltest"
    STYLE
      COLOR 255 0 0
      OUTLINECOLOR 0 0 0
    END
  END
END