Question

I’m trying to delete orphan entries in a mysql 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 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.id.

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

Answer 1

Using LEFT JOIN/IS NULL:

DELETE b 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.