Sql Query help to get non matching records from two tables

create table #one (id int,acc nvarchar(25))
insert into #one (id , acc) values(1,'one') 
insert into #one (id , acc) values(2,'two') 
insert into #one (id , acc) values(3,'three') 

create table #two (acct nvarchar(25),ids int)
insert into #two (acct,ids) values('one',1) 
insert into #two (acct,ids) values('two',3) 
insert into #two (acct,ids) values('four',4) 

select ids from #two EXCEPT select id from #one 

drop table #one 
drop table #two 

test this one

SELECT B.Accountid 
  FROM TableB AS B 
  LEFT 
  JOIN TableA AS A 
    ON A.ID = B.Accountid 
 WHERE A.ID IS NULL;

LEFT JOIN means it takes all the rows from the first table - if there are no matches on the first join condition, the result table columns for table B will be null - that's why it works.