使用自连接的方法实现行转列

CREATE TABLE `user_kills` (
  `id` int(11) NOT NULL,
  `user_id` int(11) DEFAULT NULL,
  `timestr` datetime DEFAULT NULL,
  `kills` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `user_kills` VALUES (‘1‘, ‘2‘, ‘2020-01-10 00:00:00‘, ‘10‘);
INSERT INTO `user_kills` VALUES (‘2‘, ‘2‘, ‘2020-02-01 00:00:00‘, ‘2‘);
INSERT INTO `user_kills` VALUES (‘3‘, ‘2‘, ‘2020-02-05 00:00:00‘, ‘12‘);
INSERT INTO `user_kills` VALUES (‘4‘, ‘4‘, ‘2020-01-10 00:00:00‘, ‘3‘);
INSERT INTO `user_kills` VALUES (‘5‘, ‘2‘, ‘2020-02-11 00:00:00‘, ‘5‘);
INSERT INTO `user_kills` VALUES (‘6‘, ‘2‘, ‘2020-01-06 00:00:00‘, ‘1‘);
INSERT INTO `user_kills` VALUES (‘7‘, ‘3‘, ‘2020-01-11 00:00:00‘, ‘20‘);
INSERT INTO `user_kills` VALUES (‘8‘, ‘2‘, ‘2020-02-12 00:00:00‘, ‘10‘);
INSERT INTO `user_kills` VALUES (‘9‘, ‘2‘, ‘2020-02-07 00:00:00‘, ‘17‘);

SELECT * FROM
(
SELECT sum(k.kills) as ‘猪八戒‘
FROM user_kills k  
WHERE k.user_id = 2
) a CROSS JOIN(
SELECT sum(k.kills) as ‘孙悟空‘
FROM user_kills k  
WHERE k.user_id = 3
) b CROSS JOIN(
SELECT sum(k.kills) as ‘沙悟净‘
FROM user_kills k  
WHERE k.user_id = 4
) c

SELECT 
sum(case when user_id = 2 then k.kills end) as ‘猪八戒‘,
sum(case when user_id = 3 then k.kills end) as ‘孙悟空‘,
sum(case when user_id = 4 then k.kills end) as ‘沙悟净‘
FROM user_kills k