The NullAsNull statement turns off the conversion of NULL values to string values previously set by a NullAsValue statement.
The NullAsValue statement operates as a switch and can be turned on or off several times in the script, using either a NullAsValue or a NullAsNull statement.
|A comma separated list of the fields for which NullAsNull should be turned on. Using * as field list indicates all fields. The wildcard characters * and ? are allowed in field names. Quoting of field names may be necessary when wildcards are used.
LOAD A,B from x.csv;