Fields that are totally blank cause the record to be rejected. To load one of these fields as NULL, use the NULLIF clause with the BLANKS parameter.
If an all-blank CHAR field is surrounded by enclosure delimiters, then the blanks within the enclosures are loaded. Otherwise, the field is loaded as NULL.
A DATE or numeric field that consists entirely of blanks is loaded as a NULL field.
Case study 6, Loading Data Using the Direct Path Load Method, for an example of how to load all-blank fields as NULL with the NULLIF clause. (See "SQL*Loader Case Studies" for information on how to access case studies.)
"How the PRESERVE BLANKS Option Affects Whitespace Trimming"