You can specify a NULLIF clause at the table level. The syntax is as follows:
NULLIF {=|!=}{"char_string"|x'hex_string'|BLANKS}
The char_string and hex_string values must be enclosed in either single quotation marks or double quotation marks.
This specification is used for each mapped character field unless a NULLIF clause is specified at the field level. A NULLIF clause specified at the field level overrides a NULLIF clause specified at the table level.
SQL*Loader checks the specified value against the value of the field in the record. If there is a match using the equal or not equal specification, then the field is set to NULL for that row. Any field that has a length of 0 after blank trimming is also set to NULL.
If you do not want the default NULLIF or any other NULLIF clause applied to a field, you can specify NO NULLIF at the field level.
"Using the WHEN_ NULLIF_ and DEFAULTIF Clauses" for more information about specifying a NULLIF clause at the field level