MySQL - GROUP results BY a single column alongside ORDER BY and LIMIT

Question

I want to select a list of tasks from my database. The tasks have a category_id . I want to get a singlur task per category_id . So if I, for example, had 10 task s that are linked to like 6 categories that would result in 6 results. The 6 results I want are determined by their id, the lowest id among the GROUP BY is the correct record for that GROUP . Also the maximum result set can be no larger than 20 ('LIMIT').

SELECT * FROM `task` WHERE `datetime`<NOW()  `task_status_id`=1 GROUP BY `category_id` ORDER BY `id` ASC LIMIT 20

What is wrong with the above query, I got no clue, I'm also at a loss getting any google results for this.

ADDED LATER

http://sqlfiddle.com/#!9/fa39cf

CREATE TABLE `category` (
  `id` int(10) UNSIGNED NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO `category` (`id`) VALUES
(1),
(2),
(3);


CREATE TABLE `task` (
  `id` int(10) UNSIGNED NOT NULL,
  `category_id` int(10) UNSIGNED NOT NULL,
  `task_status_id` int(10) UNSIGNED DEFAULT '1',
  `datetime` datetime NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO `task` (`id`, `category_id`, `task_status_id`, `datetime`) VALUES
(3, 2, 1, '2018-07-24 11:20:26'),
(4, 2, 1, '2018-07-24 11:20:26'),
(5, 3, 1, '2018-07-24 11:21:35'),
(6, 3, 1, '2018-07-24 11:21:35');

Answer 1

You can try first finding the smallest id for each category and then joining it with the task table to get the remaining details.

SELECT t.* FROM task t 
JOIN (SELECT category_id, min(id) id from task group by category_id) tc 
    ON (t.id = tc.id)
LIMIT 20