1、先建立基礎表，新增資料，

CREATE TABLE `t_user` (
  `user_id` int(11) NOT NULL AUTO_INCREMENT COMMENT '使用者id',
  `name` varchar(255) DEFAULT NULL COMMENT '名稱',
  `mobile` varchar(100) DEFAULT NULL COMMENT '電話',
  PRIMARY KEY (`user_id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

INSERT INTO `test`.`t_user` (`user_id`, `name`, `mobile`) VALUES ('1', '唐僧', '65651615,6111651,51651651,61565161,6156');
INSERT INTO `test`.`t_user` (`user_id`, `name`, `mobile`) VALUES ('2', '悟空', '452651651,478855,41223');
INSERT INTO `test`.`t_user` (`user_id`, `name`, `mobile`) VALUES ('3', '八戒', '325489,3214778,38955,39999');
INSERT INTO `test`.`t_user` (`user_id`, `name`, `mobile`) VALUES ('4', '沙僧', '25555,23333,21111');



CREATE TABLE `t_kill` (
`kill_id` int(11) NOT NULL AUTO_INCREMENT COMMENT 'id主鍵',
`number` int(11) DEFAULT NULL COMMENT '殺敵數',
`time` datetime DEFAULT NULL COMMENT '時間',
`user_id` int(11) DEFAULT NULL COMMENT '使用者id',
PRIMARY KEY (`kill_id`)
) ENGINE=InnoDB AUTO_INCREMENT=17 DEFAULT CHARSET=utf8;

 

INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('6', '12', '2020-12-27 15:31:23', '2');
INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('3', '5', '2020-12-26 15:31:23', '1');
INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('4', '8', '2020-12-25 15:31:23', '1');
INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('10', '15', '2020-12-24 15:31:23', '3');
INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('8', '10', '2020-12-18 15:31:23', '2');
INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('7', '52', '2020-12-08 15:31:23', '2');
INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('5', '47', '2020-11-28 15:31:23', '2');
INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('9', '8', '2020-10-27 15:31:23', '3');
INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('1', '15', '2020-10-18 15:31:23', '1');
INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('15', '11', '2020-10-17 15:31:23', '4');
INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('14', '12', '2020-10-16 15:31:23', '4');
INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('13', '21', '2020-10-15 15:31:23', '4');
INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('12', '35', '2020-10-11 15:31:23', '3');
INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('16', '9', '2020-10-10 15:31:23', '4');
INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('2', '9', '2020-09-26 15:31:23', '1');
INSERT INTO `test`.`t_kill` (`kill_id`, `number`, `time`, `user_id`) VALUES ('11', '24', '2020-09-25 15:31:23', '3');

2.列轉行， 使用 case when

-- 查詢使用者殺敵數 
SELECT
    tu.user_id,
    tu.`name`,
    SUM(tk.number) total
FROM
    t_user tu
LEFT JOIN t_kill tk ON tu.user_id = tk.user_id
GROUP BY tu.name;

-- 使用 case when
SELECT
sum(CASE WHEN tu.name = '唐僧' then tk.number end) '唐僧',
sum(CASE WHEN tu.name = '悟空' then tk.number end) '悟空',
sum(CASE WHEN tu.name = '八戒' then tk.number end) '八戒',
sum(CASE WHEN tu.name = '沙僧' then tk.number end) '沙僧'
FROM
t_user tu
LEFT JOIN t_kill tk ON tu.user_id = tk.user_id
;

3、行轉列

-- 稍等

4、分組查詢前幾條資料

-- 查詢每一條資料，cnt是t_kill表中number比當前資料number大的資料的條數
SELECT
    tu.user_id,
    tu.`name`,
    tk.kill_id,
    tk.number,
    tk.time,
    ( SELECT count(1) from t_kill t where t.number > tk.number and t.user_id = tu.user_id) cnt
FROM
    t_user tu
LEFT JOIN t_kill tk ON tu.user_id = tk.user_id;
-- 根據cnt查詢資料
SELECT * from (SELECT
    tu.user_id,
    tu.`name`,
    tk.kill_id,
    tk.number,
    tk.time,
    ( SELECT count(1) from t_kill t where t.number > tk.number and t.user_id = tu.user_id) cnt
FROM
    t_user tu
LEFT JOIN t_kill tk ON tu.user_id = tk.user_id) c 
where cnt < 2
ORDER BY name , number desc;