MySQL行转列案例分析

建表：

成绩表：表名score，列名：学号、课程号、成绩

CREATE TABLE `score` (
  `s_id` varchar(20) NOT NULL,
  `c_id` varchar(20) NOT NULL,
  `s_score` int(3) DEFAULT NULL,
  PRIMARY KEY (`s_id`,`c_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

插入数据脚本：

insert into Score values('01' , '01' , 80);
insert into Score values('01' , '02' , 90);
insert into Score values('01' , '03' , 99);
insert into Score values('02' , '01' , 70);
insert into Score values('02' , '02' , 60);
insert into Score values('02' , '03' , 80);
insert into Score values('03' , '01' , 80);
insert into Score values('03' , '02' , 80);
insert into Score values('03' , '03' , 80);
insert into Score values('04' , '01' , 50);
insert into Score values('04' , '02' , 30);
insert into Score values('04' , '03' , 20);
insert into Score values('05' , '01' , 76);
insert into Score values('05' , '02' , 87);
insert into Score values('06' , '01' , 31);
insert into Score values('06' , '03' , 34);
insert into Score values('07' , '02' , 89);
insert into Score values('07' , '03' , 98);

表数据如下图：

要求将该表行转列形成以下结构：

思路：

1. 使用常量列输出目标表的结构

SELECT s_id, '01', '02', '03' from score;

2. 使用case when表达式，替换常量列为对应的成绩

遍历score每一行，遇到不同的cid，将其值利用case/when转换成对应列的成绩

select s_id,
(case c_id when '01' THEN s_score ELSE 0 end) as '01',
(case c_id when '02' then s_score ELSE 0 end) as '02',
(case c_id when '03' then s_score ELSE 0 end) as '03'
from score;

经过上述转换后和最终结果很接近，但是s_id存在很多重复行，可以将其聚合成一行数据，这时很容易想到使用s_id分组，同时我们需要取出c_id在相同s_id下最大值，填入新的行即可

3.分组，求最大值

select s_id,
max(case c_id WHEN '01' then s_score else 0 end) '01',
max(case c_id WHEN '02' then s_score else 0 end) '02',
max(case c_id WHEN '03' then s_score else 0 end) '03'
FROM score
GROUP BY s_id

最后得到上面要求的结果：