CODEWITHSUNDEEP
sqlmysql
Martin
Martin

Reputation: 10563

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

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.

Upvotes: 223

Views: 246302

Answers (4)

Royer Adames
Royer Adames

Reputation: 1076

DELETE FROM <table> 
WHERE <row column you want to delete by > not in 
(select <column you want to compare other column> from <other table>)

This deletes the row when the first column doesn't appear on the second table

Upvotes: 2

George
George

Reputation: 6084

DELETE FROM blob
WHERE NOT EXISTS (
    SELECT *
    FROM files
    WHERE id=blob.id
)

Upvotes: 24

OMG Ponies
OMG Ponies

Reputation: 332521

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.

Upvotes: 427

Martin Smith
Martin Smith

Reputation: 452978

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...*/
      )

Upvotes: 33

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