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MySQL DELETE FROM with subquery as condition

Posted by: admin November 1, 2017 Leave a comment

Questions:

I am trying to do a query like this:

DELETE FROM term_hierarchy AS th
WHERE th.parent = 1015 AND th.tid IN (
    SELECT DISTINCT(th1.tid)
    FROM term_hierarchy AS th1
    INNER JOIN term_hierarchy AS th2 ON (th1.tid = th2.tid AND th2.parent != 1015)
    WHERE th1.parent = 1015
);

As you can probably tell, I want to delete the parent relation to 1015 if the same tid has other parents. However, that yields me a syntax error:

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'AS th
WHERE th.parent = 1015 AND th.tid IN (
  SELECT DISTINCT(th1.tid)
  FROM ter' at line 1

I have checked the documentation, and run the subquery by itself, and it all seems to check out. Can anyone figure out what’s wrong here?

Update: As answered below, MySQL does not allow the table you’re deleting from be used in a subquery for the condition.

Answers:

You cannot specify target table for delete.

A workaround

create table term_hierarchy_backup (tid int(10)); <- check data type

insert into term_hierarchy_backup 
SELECT DISTINCT(th1.tid)
FROM term_hierarchy AS th1
INNER JOIN term_hierarchy AS th2 ON (th1.tid = th2.tid AND th2.parent != 1015)
WHERE th1.parent = 1015;

DELETE FROM term_hierarchy AS th
WHERE th.parent = 1015 AND th.tid IN (select tid from term_hierarchy_backup);

Questions:
Answers:

For others that find this question looking to delete while using a subquery, I leave you this example for outsmarting MySQL (even if some people seem to think it cannot be done):

DELETE e.*
FROM tableE e
WHERE id IN (SELECT id
             FROM tableE
             WHERE arg = 1 AND foo = 'bar');

will give you an error:

ERROR 1093 (HY000): You can't specify target table 'e' for update in FROM clause

However this query:

DELETE e.*
FROM tableE e
WHERE id IN (SELECT id
             FROM (SELECT id
                   FROM tableE
                   WHERE arg = 1 AND foo = 'bar') x);

will work just fine:

Query OK, 1 row affected (3.91 sec)

Wrap your subquery up in an additional subquery (here named x) and MySQL will happily do what you ask.

Questions:
Answers:

The alias should be included after the DELETE keyword:

DELETE th
FROM term_hierarchy AS th
WHERE th.parent = 1015 AND th.tid IN 
(
    SELECT DISTINCT(th1.tid)
    FROM term_hierarchy AS th1
    INNER JOIN term_hierarchy AS th2 ON (th1.tid = th2.tid AND th2.parent != 1015)
    WHERE th1.parent = 1015
);

Questions:
Answers:

You need to refer to the alias again in the delete statement, like:

DELETE th FROM term_hierarchy AS th
....

As outlined here in MySQL docs.

Questions:
Answers:

I approached this in a slightly different way and it worked for me;

I needed to remove secure_links from my table that referenced the conditions table where there were no longer any condition rows left. A housekeeping script basically. This gave me the error – You cannot specify target table for delete.

So looking here for inspiration I came up with the below query and it works just fine.
This is because it creates a temporary table sl1 that is used as the reference for the DELETE.

DELETE FROM `secure_links` WHERE `secure_links`.`link_id` IN 
            (
            SELECT
                `sl1`.`link_id` 
            FROM 
                (
                SELECT 

                    `sl2`.`link_id` 

                FROM 
                    `secure_links` AS `sl2` 
                    LEFT JOIN `conditions` ON `conditions`.`job` = `sl2`.`job` 

                WHERE 

                    `sl2`.`action` = 'something' AND 
                    `conditions`.`ref` IS NULL 
                ) AS `sl1`
            )

Works for me.

Questions:
Answers:

Isn’t the “in” clause in the delete … where, extremely inefficient, if there are going to be a large number of values returned from the subquery? Not sure why you would not just inner (or right) join back against the original table from the subquery on the ID to delete, rather than us the “in (subquery)”.?

DELETE T FROM Target AS T
RIGHT JOIN (full subquery already listed for the in() clause in answers above) ` AS TT ON (TT.ID = T.ID)

And maybe it is answered in the “MySQL doesn’t allow it”, however, it is working fine for me PROVIDED I make sure to fully clarify what to delete (DELETE T FROM Target AS T). Delete with Join in MySQL clarifies the DELETE / JOIN issue.