一文弄懂MYSQL如何列转行

一、需求：

有三张表，学生表、成绩表和课程表，我们可以通过连表查询出学生姓名、课程及对应的成绩： 所需表sql

-- ----------------------------
-- Table structure for student
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
 `s_id` varchar(20) NOT NULL DEFAULT '',
 `s_name` varchar(20) NOT NULL DEFAULT '',
 `s_birth` varchar(20) NOT NULL DEFAULT '',
 `s_sex` varchar(10) NOT NULL DEFAULT '',
 PRIMARY KEY (`s_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES ('01', '赵雷', '1990-01-01', '男');
INSERT INTO `student` VALUES ('02', '钱电', '1990-12-21', '男');
INSERT INTO `student` VALUES ('03', '孙风', '1990-05-20', '男');
INSERT INTO `student` VALUES ('04', '李云', '1990-08-06', '男');
INSERT INTO `student` VALUES ('05', '周梅', '1991-12-01', '女');
INSERT INTO `student` VALUES ('06', '吴兰', '1992-03-01', '女');
INSERT INTO `student` VALUES ('07', '郑竹', '1989-07-01', '女');
INSERT INTO `student` VALUES ('08', '王菊', '1990-01-20', '女');

-- ----------------------------
-- Table structure for course
-- ----------------------------
DROP TABLE IF EXISTS `course`;
CREATE TABLE `course` (
 `c_id` varchar(20) NOT NULL DEFAULT '',
 `c_name` varchar(20) NOT NULL DEFAULT '',
 `t_id` varchar(20) NOT NULL,
 PRIMARY KEY (`c_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of course
-- ----------------------------
INSERT INTO `course` VALUES ('01', '语文', '02');
INSERT INTO `course` VALUES ('02', '数学', '01');
INSERT INTO `course` VALUES ('03', '英语', '03');

-- ----------------------------
-- Table structure for score
-- ----------------------------
DROP TABLE IF EXISTS `score`;
CREATE TABLE `score` (
 `s_id` varchar(20) NOT NULL DEFAULT '',
 `c_id` varchar(20) NOT NULL DEFAULT '',
 `s_score` int(3) DEFAULT NULL,
 PRIMARY KEY (`s_id`,`c_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of score
-- ----------------------------
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');

SELECT s.s_id,s.s_name,c.c_name,sc.s_score
FROM student s
LEFT JOIN score sc on sc.s_id = s.s_id
LEFT JOIN course c on c.c_id = sc.c_id

好的，现在呢我们要把课程名称呢变成横行呢？

二、如何实现

1）首先看我们的静态SQL

关联成绩表课程表查询学生各科课程成绩

SELECT s.s_id,s.s_name,c.c_name,sc.s_score
FROM student s
LEFT JOIN score sc on sc.s_id=s.s_id
LEFT JOIN course c on c.c_id = sc.c_id;

IF(s1,s2,s3)表达式,类似三木运算符取值,s1值为真取s2值,假取s3个值,最后可得到某一科成绩

SELECT  p.s_id,p.s_name, p.c_name,p.c_name = '数学',
IF(p.c_name = '数学',p.c_name,NULL)c_name,IF(p.c_name = '数学',p.s_score,NULL)s_score
FROM (
SELECT s.s_id,s.s_name,c.c_name,sc.s_score
FROM student s
LEFT JOIN score sc on sc.s_id=s.s_id
LEFT JOIN course c on c.c_id = sc.c_id)p;

然后我们分组且用MAX函数获取每个学生的数学课程的成绩,替换这一课的字段名称

SELECT  p.s_id,
       p.s_name,
       MAX(IF(p.c_name = '数学', p.s_score, NULL)) AS 数学
FROM (
SELECT s.s_id,s.s_name,c.c_name,sc.s_score
FROM student s
LEFT JOIN score sc on sc.s_id=s.s_id
LEFT JOIN course c on c.c_id = sc.c_id)p
GROUP BY p.s_id;

获取所有人各科成绩

SELECT  p.s_id,
       p.s_name,
       MAX(IF(p.c_name = '数学', p.s_score, NULL)) AS 数学,
       MAX(IF(p.c_name = '语文', p.s_score, NULL)) AS 语文,
       MAX(IF(p.c_name = '英语', p.s_score, NULL)) AS 英语
FROM (
SELECT s.s_id,s.s_name,c.c_name,sc.s_score
FROM student s
LEFT JOIN score sc on sc.s_id=s.s_id
LEFT JOIN course c on c.c_id = sc.c_id)p
GROUP BY p.s_id;

2)那么就有人问了，如果我有100门课程不是要写100次名称，这也太麻烦了？

接下来请看动态SQL

我们的动态sql是拼接实现的, 主要就是拼接我们的课程成绩那一句, 所以要先看一下CONCAT函数拼接课程语句

SELECT c_name,CONCAT( 'MAX(IF(p.c_name = ''', c_name, ''', c.s_score, NULL)) AS ', c_name ) FROM course c;

是的，结果就是上面要的MAX函数

然后我么可以用GROUP_CONCAT()函数把这些内容拼接成一句

SELECT GROUP_CONCAT(DISTINCT c_name,CONCAT( 'MAX(IF(p.c_name = ''', c_name, ''', c.s_score, NULL)) AS ', c_name )) FROM course c;

接下来，拼接sql实现需求

-- 1.定义一个sql变量
SET @sql = NULL;

-- 2.把我们的查询课程的sql赋给变量
SELECT GROUP_CONCAT(DISTINCT CONCAT('MAX(IF(p.c_name = ''',c_name,''', p.s_score, NULL)) AS ',c_name)) INTO @sql
FROM course;

-- 3.拼接sql
SET @sql = CONCAT('SELECT  p.s_id, p.s_name, ', @sql ,'
FROM (SELECT s.s_id,s.s_name,c.c_name,sc.s_score
FROM student s
LEFT JOIN score sc on sc.s_id=s.s_id
LEFT JOIN course c on c.c_id = sc.c_id)p
GROUP BY p.s_id');

-- 预处理语句
PREPARE stmt FROM @sql;
-- 执行
EXECUTE stmt;
-- 销毁
DEALLOCATE PREPARE stmt;

3)这样每次都写一长串sql也很麻烦？

好的 那么我们来封装成存储过程

-- 1、创建无参存储过程
delimiter $$
CREATE PROCEDURE getStudentRow()
BEGIN
   ------把要执行的sql放在这里就可以了
SET @sql = NULL;
SELECT
       GROUP_CONCAT(DISTINCT CONCAT('MAX(IF(p.c_name = ''',c_name,''', p.s_score, NULL)) AS ',c_name))
       INTO @sql FROM course;
   SET @sql = CONCAT('SELECT  p.s_id, p.s_name, ', @sql ,'
   FROM (SELECT s.s_id,s.s_name,c.c_name,sc.s_score
   FROM student s
   LEFT JOIN score sc on sc.s_id=s.s_id
   LEFT JOIN course c on c.c_id = sc.c_id)p
   GROUP BY p.s_id');
   PREPARE stmt FROM @sql;
   EXECUTE stmt;
   DEALLOCATE PREPARE stmt;
   ------把要执行的sql放在这里就可以了
END$$;
delimiter;

-- 查询存储过程
SHOW PROCEDURE STATUS;

-- 调用
CALL getStudentRow();

这样每次直接调用就可以了？

来源：https://blog.csdn.net/qq_42145865/article/details/108186843