What's the difference between 'not in' and 'not exists'?

Question

What\'s the difference between not in and not exists in an Oracle query?

When do I use not in? And not exist?

Answer 1

The difference between NOT IN and NOT EXISTS becomes clear where there are NULL values included in the result.

For example:

create table test_a (col1 varchar2(30 char));
create table test_b (col1 varchar2(30 char));

insert into test_a (col1) values ('a');
insert into test_a (col1) values ('b');
insert into test_a (col1) values ('c');
insert into test_a (col1) values ('d');
insert into test_a (col1) values ('e');

insert into test_b (col1) values ('a');
insert into test_b (col1) values ('b');
insert into test_b (col1) values ('c');
insert into test_b (col1) values (null);

Note: They key difference is that test_b contains a null value.

select * from test_a where col1 not in (select col1 from test_b);

No rows returned

select * from test_a where 
    not exists
        (select 1 from test_b where test_b.col1 = test_a.col1);

Returns

col1
====
d
e