TP

tp.sql

-- create table tmp2(
-- foo int
-- )

-- insert into tmp values (1),(2),(4),(4),(5),(null);
-- insert into tmp2 values (1),(3),(4),(4),(5),(5),(null);
-- select tmp.foo, tmp2.foo 
-- from tmp
-- inner join tmp2 on tmp.foo = tmp2.foo


--  create table emp(
--  id int,
--  chef_id int,
--  dept int
--  );
 
--  insert into emp values
--  (1,null,1),
--  (2,1,1),
--  (3,1,2),
--  (4,2,3),
--  (5,3,1),
--  (6,1,2);
 
-- select *
-- from emp a
-- left join emp b on b.id = a.chef_id and b.dept = a.dept
-- where b.id is null 

-- select *
-- from emp a
-- left join emp b on b.id = a.chef_id
-- where b.id is not null