一文弄懂MYSQL如何列轉行

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一、需求：

有三張表，學生表、成績表和課程表，我們可以通過連表查詢出學生姓名、課程及對應的成績： 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', '趙雷', '1990-01-01', '男');
INSERT INTO `student` VALUES ('02', '錢電', '1990-12-21', '男');
INSERT INTO `student` VALUES ('03', '孫風', '1990-05-20', '男');
INSERT INTO `student` VALUES ('04', '李雲', '1990-08-06', '男');
INSERT INTO `student` VALUES ('05', '周梅', '1991-12-01', '女');
INSERT INTO `student` VALUES ('06', '吳蘭', '1992-03-01', '女');
INSERT INTO `student` VALUES ('07', '鄭竹', '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', '語文', '02');
INSERT INTO `course` VALUES ('02', '數學', '01');
INSERT INTO `course` VALUES ('03', '英語', '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，現在呢我們要把課程名稱呢變成橫行呢？

二、如何實現

1）首先看我們的靜態SQL

關聯成績表課程表查詢學生各科課程成績

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)表達式,類似三木運算符取值,s1值為真取s2值,假取s3個值,最後可得到某一科成績

SELECT  p.s_id,p.s_name, p.c_name,p.c_name = '數學',
IF(p.c_name = '數學',p.c_name,NULL)c_name,IF(p.c_name = '數學',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;

然後我們分組且用MAX函數獲取每個學生的數學課程的成績,替換這一課的字段名稱

SELECT  p.s_id,
        p.s_name, 
        MAX(IF(p.c_name = '數學', p.s_score, NULL)) AS 數學
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;

獲取所有人各科成績

SELECT  p.s_id,
        p.s_name, 
        MAX(IF(p.c_name = '數學', p.s_score, NULL)) AS 數學,
        MAX(IF(p.c_name = '語文', p.s_score, NULL)) AS 語文,
        MAX(IF(p.c_name = '英語', p.s_score, NULL)) AS 英語
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)那麼就有人問瞭，如果我有100門課程不是要寫100次名稱，這也太麻煩瞭？

接下來請看動態SQL

我們的動態sql是拼接實現的, 主要就是拼接我們的課程成績那一句, 所以要先看一下CONCAT函數拼接課程語句

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

是的，結果就是上面要的MAX函數

然後我麼可以用GROUP_CONCAT()函數把這些內容拼接成一句

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

接下來，拼接sql實現需求

-- 1.定義一個sql變量
SET @sql = NULL;
 
-- 2.把我們的查詢課程的sql賦給變量
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');
 
-- 預處理語句 
PREPARE stmt FROM @sql;
-- 執行
EXECUTE stmt;
-- 銷毀
DEALLOCATE PREPARE stmt;

3)這樣每次都寫一長串sql也很麻煩？

好的 那麼我們來封裝成存儲過程

-- 1、創建無參存儲過程
delimiter $$
CREATE PROCEDURE getStudentRow()
BEGIN
    ------把要執行的sql放在這裡就可以瞭
		SET @sql = NULL;
		SELECT
        GROUP_CONCAT(DISTINCT CONCAT('MAX(IF(p.c_name = ''',c_name,''', p.s_score, NULL)) AS ',c_name)) 
        INTO @sql FROM course;
    SET @sql = CONCAT('SELECT  p.s_id, p.s_name, ', @sql ,'
    	FROM (SELECT s.s_id,s.s_name,c.c_name,sc.s_score 
    	FROM student s 
    	LEFT JOIN score sc on sc.s_id=s.s_id 
    	LEFT JOIN course c on c.c_id = sc.c_id)p 
    	GROUP BY p.s_id');
    PREPARE stmt FROM @sql;
    EXECUTE stmt;
    DEALLOCATE PREPARE stmt;
    ------把要執行的sql放在這裡就可以瞭
END$$;
delimiter;
 
-- 查詢存儲過程
SHOW PROCEDURE STATUS;
 
-- 調用
CALL getStudentRow();

這樣每次直接調用就可以瞭？

總結

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