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一文弄懂MYSQL如何列转行

来源：脚本之家 2022-12-29 17:52:45 0浏览收藏

本篇文章主要是结合我之前面试的各种经历和实战开发中遇到的问题解决经验整理的，希望这篇《一文弄懂MYSQL如何列转行》对你有很大帮助！欢迎收藏，分享给更多的需要的朋友学习~

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();