Not in VS Exists VS Except

-- not in
-- if the cust_id column is nullable then null value will return true on the next query !!!!!!!!!!!!!
select * from table where customer_id not in (select cust_id from cust);

-- left join or not exists

left join can be a performance problem.
use not exists it is faster

-- examples

declare @x table(a int);
insert @x values(1),(1),(null);

declare @y table(b int not null);
insert @y values(1),(1),(2),(2);

-- bad use
-- where 1 <> 1 = false
-- where 2 <> null = unknown = false
select b from @y where b not in (select a from @x);

-- best use
select b from @y as y where not exists 
    (select 1 from @x as x
      where x.a = y.b);

-- return distinct values after ordered.
-- performance problem too.
select b from @y
except
select a from @x;