select a value where it doesn't exist in another table

Question

I have two tables

Table A:

ID
1
2
3
4

Table B:

ID
1
2
3

I have two requests:

• I want to

Answer 1

select ID from A where ID not in (select ID from B);

or

select ID from A except select ID from B;

Your second question:

delete from A where ID not in (select ID from B);