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

For your first question there are at least three common methods to choose from:

• NOT EXISTS
• NOT IN
• LEFT JOIN

The SQL looks like this:

SELECT * FROM TableA WHERE NOT EXISTS (
    SELECT NULL
    FROM TableB
    WHERE TableB.ID = TableA.ID
)

SELECT * FROM TableA WHERE ID NOT IN (
    SELECT ID FROM TableB
)

SELECT TableA.* FROM TableA 
LEFT JOIN TableB
ON TableA.ID = TableB.ID
WHERE TableB.ID IS NULL

Depending on which database you are using, the performance of each can vary. For SQL Server (not nullable columns):

NOT EXISTS and NOT IN predicates are the best way to search for missing values, as long as both columns in question are NOT NULL.

Answer 2

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);

Answer 3

This would select 4 in your case

SELECT ID FROM TableA WHERE ID NOT IN (SELECT ID FROM TableB)

This would delete them

DELETE FROM TableA WHERE ID NOT IN (SELECT ID FROM TableB)

Answer 4

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) 

Answer 5

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

Answer 6

You could use NOT IN:

SELECT A.* FROM A WHERE ID NOT IN(SELECT ID FROM B)

However, meanwhile i prefer NOT EXISTS:

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

There are other options as well, this article explains all advantages and disadvantages very well:

Should I use NOT IN, OUTER APPLY, LEFT OUTER JOIN, EXCEPT, or NOT EXISTS?