Creates a global field definition.

    When the DEFINE FIELD statement executes, Oracle Rdb adds the field
    definition to the physical database. A field definition includes
    a name, a data type, and a set of optional clauses.

    Example:

    DEFINE FIELD LABEL
       DATATYPE IS TEXT SIZE IS 8 CHARACTERS
       MISSING_VALUE IS "Not here"
       VALID IF LABEL =  "Good"       OR
                LABEL =  "Bad"        OR
                LABEL =  "Mediocre"   OR
                LABEL MISSING
       QUERY_HEADER FOR DATATRIEVE IS "Column"/"Label".