load data
infile *
into table test_tab
fields
(col1 position(1:1),
col2 position(2:6) PRESERVE BLANKS,
col3 position(7:7))
begindata:
1     7
7     1

SCOTT@orcl_11gR2> create table test_tab
  2    (col1  number,
  3  	col2  char(5),
  4  	col3  number)
  5  /

Table created.

SCOTT@orcl_11gR2> host sqlldr scott/tiger control=test.ctl log=test.log

SCOTT@orcl_11gR2> select col1, col2, col3, length (col2) from test_tab
  2  /

         1                7            5
         7                1            5

2 rows selected.

SCOTT@orcl_11gR2> select * from test_tab where col2 is null
  2  /

no rows selected

SCOTT@orcl_11gR2>