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set null [SQL*Plus]

set null some-string
set null defines how nulls are displayed in a result set: 
RENE@ora10> create table null_demo (a number, b number);

Table created.

RENE@ora10> insert into null_demo values(1,2);

1 row created.

RENE@ora10> insert into null_demo values(null, 5);

1 row created.

RENE@ora10> insert into null_demo values(7, null);

1 row created.

RENE@ora10> select * from null_demo;

         A          B
---------- ----------
         1          2
                    5
         7

RENE@ora10> set null <null>
RENE@ora10> select * from null_demo;

         A          B
---------- ----------
         1          2
<null>              5
         7 <null>