Syntax:
FIELD(str,str1,str2,str3,...)

Returns the index (position) of str in the str1, str2, str3, ... list.
Returns 0 if str is not found.

If all arguments to FIELD() are strings, all arguments are compared as
strings. If all arguments are numbers, they are compared as numbers.
Otherwise, the arguments are compared as double.

If str is NULL, the return value is 0 because NULL fails equality
comparison with any value. FIELD() is the complement of ELT().

URL: http://dev.mysql.com/doc/refman/5.1/en/string-functions.html

