Question

Given two tables, one for workers and one for tasks completed by workers,

CREATE TABLE IF NOT EXISTS `workers` (   `id` int(11) NOT NULL,   PRIMARY KEY  (`id`) );  INSERT INTO `workers` (`id`) VALUES (1);   CREATE TABLE IF NOT EXISTS `tasks` (   `id` int(11) NOT NULL,   `worker_id` int(11) NOT NULL,   `status` int(11) NOT NULL,   PRIMARY KEY  (`id`) );  INSERT INTO `tasks` (`id`, `worker_id`, `status`) VALUES (1, 1, 1), (2, 1, 1), (3, 1, 2), (4, 1, 2), (5, 1, 2); 

I’m trying to get the number of tasks each worker has with each status code.

I can say either

SELECT w.* ,COUNT(t1.worker_id) as status_1_count FROM workers w LEFT JOIN tasks t1 ON w.id = t1.worker_id AND t1.status = 1  WHERE 1  GROUP BY  t1.worker_id ORDER BY w.id 

or

SELECT w.* ,COUNT(t2.worker_id) as status_2_count FROM workers w LEFT JOIN tasks t2 ON w.id = t2.worker_id AND t2.status = 2 WHERE 1  GROUP BY  t2.worker_id ORDER BY w.id 

and get the number of tasks with a single given status code, but when I try to get the counts for multiple task statuses in a single query, it doesn’t work!

SELECT w.* ,COUNT(t1.worker_id) as status_1_count ,COUNT(t2.worker_id) as status_2_count FROM workers w LEFT JOIN tasks t1 ON w.id = t1.worker_id AND t1.status = 1  LEFT JOIN tasks t2 ON w.id = t2.worker_id AND t2.status = 2 WHERE 1  GROUP BY t1.worker_id ,t2.worker_id ORDER BY w.id 

The tasks table is cross-joining against itself when I would rather it wouldn’t!

Is there any way to combine these two queries into one such that we can retrieve the counts for multiple task statuses in a single query?

Thanks!

Answer 1

SELECT w.*,   SUM(t1.status = 1) AS status_1_count,   SUM(t1.status = 2) AS status_2_count FROM workers w   LEFT JOIN tasks t1 ON w.id = t1.worker_id AND t1.status IN (1, 2)  GROUP BY w.id ORDER BY w.id;