Delete sql rows where IDs do not have a match in another table

Question

Hi, please can I have some help on this. I'm trying to delete orphan entries in a table.

I have 2 tables like this:

table files:

| id | ....
------------
| 1  | ....
| 2  | ....
| 7  | ....
| 9  | ....

table blob:

| fileid | ....
------------
| 1  | ....
| 2  | ....
| 3  | ....
| 4  | ....
| 4  | ....
| 4  | ....
| 9  | ....

The tables fileid and id columns can be used to join the tables together.

I want to delete all rows in table blob where fileid cannot be found in the table files.

So using the example above that would delete rows: 3 & 4(s) in the blob table.

Thanks for any help.

Answer 1

DELETE FROM blob 
WHERE fileid NOT IN 
       (SELECT id 
        FROM files 
        WHERE id is NOT NULL/*This line is unlikely to be needed 
                               but using NOT IN...*/
      )

Answer 2

Using LEFT JOIN/IS NULL:

DELETE FROM BLOB b 
  LEFT JOIN FILES f ON f.id = b.fileid 
      WHERE f.id IS NULL

Using NOT EXISTS:

DELETE FROM BLOB 
 WHERE NOT EXISTS(SELECT NULL
                    FROM FILES f
                   WHERE f.id = fileid)

Using NOT IN:

DELETE FROM BLOB
 WHERE fileid NOT IN (SELECT f.id 
                        FROM FILES f)

Warning

Whenever possible, perform DELETEs within a transaction (assuming supported - IE: Not on MyISAM) so you can use rollback to revert changes in case of problems.