Question

I have the following database structure:

Sites table

id  |  name  |  other_fields

Backups table

id  |  site_id  | initiated_on(unix timestamp) | size(float) | status

So Backups table have a Many to One relationship with Sites table connected via site_id

---

And I would like to output the data in the following format

name | Latest initiated_on | status of the latest initiated_on row

And I have the following SQL query

SELECT *, `sites`.`id` as sid, SUM(`backups`.`size`) AS size
FROM (`sites`)
LEFT JOIN `backups` ON `sites`.`id` = `backups`.`site_id`
WHERE `sites`.`id` =  '1'
GROUP BY `sites`.`id`
ORDER BY `backups`.`initiated_on` desc

The thing is, with the above query I can achieve what I am looking for, but the only problem is I don’t get the latest initiated_on values.

So if I had 3 rows in backups with site_id=1, the query does not pick out the row with the highest value in initiated_on. It just picks out any row.

Please help, and

thanks in advance.

Answer 1

You should try:

SELECT sites.name, FROM_UNIXTIME(b.latest) as latest, b.size, b.status
FROM sites
LEFT JOIN
  ( SELECT bg.site_id, bg.latest, bg.sizesum AS size, bu.status
    FROM
      ( SELECT site_id, MAX(initiated_on) as latest, SUM(size) as sizesum
        FROM backups
        GROUP BY site_id ) bg
    JOIN backups bu
    ON bu.initiated_on = bg.latest AND bu.site_id = bg.site_id
  ) b
ON sites.id = b.site_id

1. In the GROUP BY subquery – bg here, the only columns you can use for SELECT are columns that are either aggregated by a function or listed in the GROUP BY part.

   http://dev.mysql.com/doc/refman/5.5/en/group-by-hidden-columns.html

2. Once you have all the aggregate values you need to join the result again to backups to find other values for the row with latest timestamp – b.

3. Finally join the result to the sites table to get names – or left join if you want to list all sites, even without a backup.