Михаил Ливач

CREATE TABLE `items` (
  `department_id` int(10) unsigned NOT NULL,
  `type_id` int(10) unsigned NOT NULL,
  `item_id` int(10) unsigned NOT NULL,
  `item` varchar(255) NOT NULL,
  PRIMARY KEY (`department_id`,`type_id`,`item_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

INSERT INTO `items` (`department_id`, `type_id`, `item_id`, `item`) VALUES
(1,	1,	1,	'1_1_0001'),
(1,	1,	2,	'1_1_0002'),
(1,	2,	1,	'1_2_0001'),
(1,	3,	1,	'1_3_0001'),
(1,	3,	2,	'1_3_0002'),
(1,	3,	3,	'1_3_0003'),
(1,	3,	4,	'1_3_0004'),
(1,	3,	5,	'1_3_0005'),
(2,	1,	1,	'2_1_0001'),
(2,	2,	1,	'2_2_0001'),
(2,	3,	1,	'2_3_0001');

SELECT t.department_id, 
IFNULL( t1.item, '' ) as type_1,
IFNULL( t2.item, '' ) as type_2,
IFNULL( t3.item, '' ) as type_3,
IFNULL( t4.item, '' ) as type_4
FROM `items` t
LEFT JOIN `items` t1 ON ( t.department_id=t1.department_id AND t.item_id=t1.item_id AND t1.type_id=1 )
LEFT JOIN `items` t2 ON ( t.department_id=t2.department_id AND t.item_id=t2.item_id AND t2.type_id=2 )
LEFT JOIN `items` t3 ON ( t.department_id=t3.department_id AND t.item_id=t3.item_id AND t3.type_id=3 )
LEFT JOIN `items` t4 ON ( t.department_id=t4.department_id AND t.item_id=t4.item_id AND t4.type_id=4 )
GROUP BY t.department_id, t.item_id

func = function (arr) {
    return [].concat( ...arr );
};

func = function (arr) {
    let tmp = [];
    return arr.map( (item => tmp = tmp.concat(item)) ).pop();
};