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

Solution 1:

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?

Solution 2:

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.

Solution 3:

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);

Solution 4:

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)