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);

---

SELECT ID    
  FROM A a
 WHERE NOT EXISTS (
      SELECT 1 
        FROM B b
       WHERE b.ID = a.ID)

---

         SELECT a.ID 
           FROM A a    
LEFT OUTER JOIN B b 
             ON a.ID = b.ID    
          WHERE b.ID IS NULL

---

DELETE 
  FROM A 
 WHERE ID NOT IN (
      SELECT ID 
        FROM B)