In T164973#3312787, @Halfak wrote:

Talking to @Zppix in chat, I came up with the following query to quickly check for campaign completion:

SELECT 1 FROM task 
INNER JOIN campaign ON campaign_id = campaign.id 
LEFT JOIN label ON task_id = task.id 
WHERE campaign.id = <campaign ID number> 
GROUP BY task.id 
HAVING COUNT(label.data) < MAX(campaign.labels_per_task) 
LIMIT 1

If it returns a row, then the campaign is not complete. If it can't return a row, the campaign is complete.