Found this on page 458 of "MySQL Reference Manual":
If you retrieve an ENUM in a numeric context, you get the index value. Example:
mysql> SELECT enum_col+0 FROM tbl_name WHERE...
If you store a number into an ENUM, the number is treated as an index (Not in LOAD DATA, then treats all as strings). Example:
mysql> UPDATE tbl_name SET enum_col=1 WHERE...
Note: Empty enum_col returns 0, and index of null returns NULL.
Good luck!