MYSQL之如何列转行

 

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：

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，该怎么做呢？

 

 

二、如何实现

1）首先看我们的静态SQL

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，所以要先看一下

SELECT CONCAT( 'MAX(IF(p.c_name = ''', s_name, ''', p.s_score, NULL)) AS ', s_name ) FROM student S;

是的，结果就是上面要的MAX函数

 

 

 

接下来，拼接sql实现需求

--1.定义一个sql变量
SET @sql = NULL;

--2.查询所有要列转行的变量
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)你说啥？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();

 

三、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');

 

 

这样每次直接调用就可以了，你学费了吗？