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

There can be performance differences, with exists being faster.

The most important difference is the handling of nulls. Your query might seem to work the same with both in and exists, but when your sub-query returns null you might get a shock.

You might find that the existence of nulls causes exists to fail.

See Joe Celko's 'SQL for smarties' for a better explanation of when to use each.

Answer 2

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

Answer 3

I think it serves the same purpose.

not in can also take literal values whereas not exists need a query to compare the results with.

EDIT: not exists could be good to use because it can join with the outer query & can lead to usage of index, if the criteria uses column that is indexed.

EDIT2: See this question as well.

EDIT3: Let me take the above things back.
See this link. I think, it all depends on how the DB translates this & on database/indexes etc.

Answer 4

Not in is testing for the present of an element in a set of elements, so it is simpler.

Not exists can handle more complicated queries, including grouping (eg having sum(x)=z or having count(*)>3), results with multiple conditions (eg matching multiple elements), and can take advantage of indexes.

In some situations not in is easier to do than not exists. I generally find this is where I am testing for the value of a key field in set of values.

As a rule of the thumb, I prefer not exists as it covers a lot more situations than not in. Not exists can be used for every situation that not in is used for, but not the reverse.