SQL 列转行 行转列 的方法

表结构

-- 创建表
CREATE TABLE `col_to_row` (
  `ID` int(10) AUTO_INCREMENT,
  `USER_NAME` varchar(20) DEFAULT NULL,
  `COURSE` varchar(20) DEFAULT NULL,
  `SCORE` float DEFAULT '0',
  PRIMARY KEY (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

表数据

-- 插入数据
INSERT INTO col_to_row (user_name,course,score)values('张三', '语文', 58);
INSERT INTO col_to_row (user_name,course,score)values('张三', '数学', 34);
INSERT INTO col_to_row (user_name,course,score)values('张三', '英语', 58);
INSERT INTO col_to_row (user_name,course,score)values('李四', '语文', 87);
INSERT INTO col_to_row (user_name,course,score)values('李四', '数学', 45);
INSERT INTO col_to_row (user_name,course,score)values('李四', '英语', 45);
INSERT INTO col_to_row (user_name,course,score)values('王五', '语文', 34);
INSERT INTO col_to_row (user_name,course,score)values('王五', '数学', 76);
INSERT INTO col_to_row (user_name,course,score)values('王五', '英语', 89);

-- 列转行
-- 方法一
SELECT 
	user_name ,
    MAX(CASE course WHEN '语文' THEN score ELSE 0 END ) 语文,
    MAX(CASE course WHEN '数学' THEN score ELSE 0 END ) 数学,
    MAX(CASE course WHEN '英语' THEN score ELSE 0 END ) 英语
FROM col_to_row
GROUP BY user_name;

MAX的作用 当我们执行如下代码时 未加MAX

SELECT 
user_name,
(CASE course WHEN '数学' THEN score else 0 end) as 数学,
(CASE course WHEN '语文' THEN score else 0 end) as 语文,
(CASE course WHEN '英语' THEN score else 0 end) as 英语
FROM col_to_row ;

结果如下:

当添加MAX后

实际上MAX可以换成其他一些聚合函数如MIN

--方法二
SELECT 
user_name,
(SELECT score FROM col_to_row b WHERE course='数学' AND a.user_name = b.user_name) as 数学,
(SELECT score FROM col_to_row b WHERE course='语文' AND a.user_name = b.user_name) as 语文,
(SELECT score FROM col_to_row b WHERE course='英语' AND a.user_name = b.user_name) as 英语
FROM col_to_row a GROUP BY user_name;

结果