Difficult MySQL Update Query with Self-Join

Question

Our website has listings. We use a connections table with the following structure to connect our members with these various listings:

CREATE TABLE `connections` (
  `cid1` int(9) unsigned NOT NULL DEFAULT '0',
  `cid2` int(9) unsigned NOT NULL DEFAULT '0',
  `type` char(2) NOT NULL,
  `created` datetime DEFAULT NULL,
  `updated` datetime DEFAULT NULL,
  PRIMARY KEY (`cid1`,`cid2`,`type`,`cid3`),
  KEY `cid1` (`cid1`,`type`),
  KEY `cid2` (`cid2`,`type`)
);

The problem we've run into is when we have to combine duplicate listings from time to time we also need to update our member connections and have been using the following query which breaks if a member is connected to both listings:

update connections set cid2=100000
where type IN ('MC','MT','MW') AND cid2=100001;

What I can't figure out is how to do the following which would solve this issue:

update connections set cid2=100000
where type IN ('MC','MT','MW') AND cid2=100001 AND cid1 NOT IN (
    select cid1 from connections
    where type IN ('MC','MT','MW') AND cid2=100000
);

When I try to run that query I get the following error:

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

Here is some sample data. Notice the update conflict for cid1 = 10025925

+----------+--------+------+---------------------+---------------------+
| cid1     | cid2   | type | created             | updated             |
+----------+--------+------+---------------------+---------------------+
| 10010388 | 100000 | MC   | 2010-08-05 18:04:51 | 2011-06-16 16:26:17 |
| 10025925 | 100000 | MC   | 2010-10-31 09:21:25 | 2010-10-31 16:21:25 |
| 10027662 | 100000 | MC   | 2011-06-13 16:31:12 | NULL                |
| 10038375 | 100000 | MW   | 2011-02-05 05:32:35 | 2011-02-05 19:51:58 |
| 10065771 | 100000 | MW   | 2011-04-24 17:06:35 | NULL                |
| 10025925 | 100001 | MC   | 2010-10-31 09:21:45 | 2010-10-31 16:21:45 |
| 10034884 | 100001 | MC   | 2011-01-20 18:54:51 | NULL                |
| 10038375 | 100001 | MC   | 2011-02-04 05:00:35 | NULL                |
| 10041989 | 100001 | MC   | 2011-02-26 09:33:18 | NULL                |
| 10038259 | 100001 | MC   | 2011-05-07 13:34:20 | NULL                |
| 10027662 | 100001 | MC   | 2011-06-13 16:33:54 | NULL                |
| 10030855 | 100001 | MT   | 2010-12-31 20:40:18 | NULL                |
| 10038375 | 100001 | MT   | 2011-02-04 05:00:36 | NULL                |
+----------+--------+------+---------------------+---------------------+

I'm hoping that someone can suggest the right way to run the above query. Thanks in advance!

Answer 1

One possible way would be to use a temporary table for your subquery then select from the temp table. Efficiency could break down fast if you need to execute a lot of these queires though.

create temporary table subq as select cid1 from connections where type IN ('MC','MT','MW') AND cid2=100000

And

update connections set cid2=100000 where type IN ('MC','MT','MW') AND cid2=100001 AND cid1 NOT IN (select cid1 from subq);

Answer 2

The reason for the error in your query is because in MySQL you cannot SELECT from the table you are trying to UPDATE in the same query.

Use UPDATE IGNORE to avoid duplicate conflicts.

I think you should try reading INSERT ON DUPLICATE KEY. The idea is that you frame an INSERT query that always create a DUPLICATE conflict and then the UPDATE part will do its part.

Answer 3

I was thinking something like the following, but I'm not 100% sure how your data looks before and after to determine accuracy. The idea is to join the table to itself on your subquery's where clause and the exclusion where cid1 must not match.

update connections c1 left outer join connections c2
 on (c2.cid2 = 100000 and c2.type in ('MC','MT','MW') and c1.cid1 != c2.cid1)
 set c1.cid2 = 100000
 where c1.type in ('MC', 'MT', 'MW') and c1.cid2=100001 and c2.cid1 is null;

As near as I can tell, it'll work. I used your create table (minus the cid3 in the primary key) and made sure I had 2 rows with the same cid1 and different cid2 (one 100000 and the other as 100001) and that the statement only affected 1 row.

Answer 4

UPDATE connections cn1
LEFT JOIN connections cn2 ON cn1.cid1 != cn2.cid1
    AND cn2.type IN ('MC','MT','MW')
    AND cn2.cid2=100000
SET cn1.cid2=100000
WHERE cn1.TYPE IN ('MC','MT','MW') 
    AND cn1.cid2=100001 
    AND cn2.cid1 IS NULL -- i.e. there is no matching record