SQL行、列之间转换

一、行转列

即将原本同一列下多行的不同内容作为多个字段，输出对应内容。

建表语句

DROP TABLE IF EXISTS tb_score;

CREATE TABLE tb_score
(
    id      INT(11)     NOT NULL auto_increment,
    userid  VARCHAR(20) NOT NULL COMMENT '用户id',
    subject VARCHAR(20) COMMENT '科目',
    score   DOUBLE COMMENT '成绩',
    PRIMARY KEY (id)
) ENGINE = INNODB
  DEFAULT CHARSET = utf8;

插入数据

INSERT INTO tb_score(userid, subject, score)
VALUES ('001', '语文', 90);
INSERT INTO tb_score(userid, subject, score)
VALUES ('001', '数学', 92);
INSERT INTO tb_score(userid, subject, score)
VALUES ('001', '英语', 80);
INSERT INTO tb_score(userid, subject, score)
VALUES ('002', '语文', 88);
INSERT INTO tb_score(userid, subject, score)
VALUES ('002', '数学', 90);
INSERT INTO tb_score(userid, subject, score)
VALUES ('002', '英语', 75.5);
INSERT INTO tb_score(userid, subject, score)
VALUES ('003', '语文', 70);
INSERT INTO tb_score(userid, subject, score)
VALUES ('003', '数学', 85);
INSERT INTO tb_score(userid, subject, score)
VALUES ('003', '英语', 90);
INSERT INTO tb_score(userid, subject, score)
VALUES ('003', '政治', 82);

查询数据表中的内容（即转换前的结果）

								SELECT * FROM tb_score

先来看一下转换后的结果：

可以看出，这里行转列是将原来的subject字段的多行内容选出来，作为结果集中的不同列，并根据userid进行分组显示对应的score。

1、使用case…when…then 进行行转列

SELECT userid,
       SUM(CASE `subject` WHEN '语文' THEN score ELSE 0 END) as '语文',
       SUM(CASE `subject` WHEN '数学' THEN score ELSE 0 END) as '数学',
       SUM(CASE `subject` WHEN '英语' THEN score ELSE 0 END) as '英语',
       SUM(CASE `subject` WHEN '政治' THEN score ELSE 0 END) as '政治'
FROM tb_score
GROUP BY userid

2、使用IF() 进行行转列：

SELECT userid,
       SUM(IF(`subject` = '语文', score, 0)) as '语文',
       SUM(IF(`subject` = '数学', score, 0)) as '数学',
       SUM(IF(`subject` = '英语', score, 0)) as '英语',
       SUM(IF(`subject` = '政治', score, 0)) as '政治'
FROM tb_score
GROUP BY userid

注意点：

（1）SUM() 是为了能够使用GROUP BY根据userid进行分组，因为每一个userid对应的subject="语文"的记录只有一条，所以SUM() 的值就等于对应那一条记录的score的值。

假如userid =‘001’ and subject=‘语文’ 的记录有两条，则此时SUM() 的值将会是这两条记录的和，同理，使用Max()的值将会是这两条记录里面值最大的一个。但是正常情况下，一个user对应一个subject只有一个分数，因此可以使用SUM()、MAX()、MIN()、AVG()等聚合函数都可以达到行转列的效果。

（2）IF(subject=‘语文’,score,0) 作为条件，即对所有subject='语文’的记录的score字段进行SUM()、MAX()、MIN()、AVG()操作，如果score没有值则默认为0。

3、利用SUM(IF()) 生成列 + WITH ROLLUP 生成汇总行,并利用 IFNULL将汇总行标题显示为Total

SELECT IFNULL(userid, 'total')                AS userid,
       SUM(IF(`subject` = '语文', score, 0))    AS 语文,
       SUM(IF(`subject` = '数学', score, 0))    AS 数学,
       SUM(IF(`subject` = '英语', score, 0))    AS 英语,
       SUM(IF(`subject` = '政治', score, 0))    AS 政治,
       SUM(IF(`subject` = 'total', score, 0)) AS total
FROM (
         SELECT userid, IFNULL(`subject`, 'total') AS `subject`, SUM(score) AS score
         FROM tb_score
         GROUP BY userid, `subject`
         WITH ROLLUP
         HAVING userid IS NOT NULL
     ) AS A
GROUP BY userid
WITH ROLLUP;

运行结果：

4、利用SUM(IF()) 生成列 + UNION 生成汇总行,并利用 IFNULL将汇总行标题显示为 Total

SELECT userid,
       SUM(IF(`subject` = '语文', score, 0)) AS 语文,
       SUM(IF(`subject` = '数学', score, 0)) AS 数学,
       SUM(IF(`subject` = '英语', score, 0)) AS 英语,
       SUM(IF(`subject` = '政治', score, 0)) AS 政治,
       SUM(score)                          AS TOTAL
FROM tb_score
GROUP BY userid
UNION
SELECT 'TOTAL',
       SUM(IF(`subject` = '语文', score, 0)) AS 语文,
       SUM(IF(`subject` = '数学', score, 0)) AS 数学,
       SUM(IF(`subject` = '英语', score, 0)) AS 英语,
       SUM(IF(`subject` = '政治', score, 0)) AS 政治,
       SUM(score)
FROM tb_score

运行结果：

5、利用SUM(IF()) 生成列，直接生成结果不再利用子查询

SELECT IFNULL(userid, 'TOTAL')             AS userid,
       SUM(IF(`subject` = '语文', score, 0)) AS 语文,
       SUM(IF(`subject` = '数学', score, 0)) AS 数学,
       SUM(IF(`subject` = '英语', score, 0)) AS 英语,
       SUM(IF(`subject` = '政治', score, 0)) AS 政治,
       SUM(score)                          AS TOTAL
FROM tb_score
GROUP BY userid
WITH ROLLUP;

运行结果：

6、动态，适用于列不确定情况

SET @EE = '';
select @EE := CONCAT(@EE, 'sum(if(subject= \'', subject, '\',score,0)) as ', subject, ',') AS aa
FROM (SELECT DISTINCT subject FROM tb_score) A;

SET @QQ = CONCAT('select ifnull(userid,\'TOTAL\')as userid,', @EE,
                 ' sum(score) as TOTAL from tb_score group by userid WITH ROLLUP');
-- SELECT @QQ;

PREPARE stmt FROM @QQ;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

运行结果：

7、合并字段显示：利用group_concat()

SELECT userid, GROUP_CONCAT(`subject`, ":", score) AS 成绩
FROM tb_score
GROUP BY userid

运行结果：

group_concat()，手册上说明:该函数返回带有来自一个组的连接的非NULL值的字符串结果。
比较抽象，难以理解。通俗点理解，其实是这样的：group_concat()会计算哪些行属于同一组，将属于同一组的列显示出来。要返回哪些列，由函数参数(就是字段名)决定。分组必须有个标准，就是根据group by指定的列进行分组。

结论：group_concat()函数可以很好的建属于同一分组的多个行转化为一个列。

三、列转行

建表语句：

CREATE TABLE tb_score1
(
    id         INT(11)     NOT NULL auto_increment,
    userid     VARCHAR(20) NOT NULL COMMENT '用户id',
    cn_score   DOUBLE COMMENT '语文成绩',
    math_score DOUBLE COMMENT '数学成绩',
    en_score   DOUBLE COMMENT '英语成绩',
    po_score   DOUBLE COMMENT '政治成绩',
    PRIMARY KEY (id)
) ENGINE = INNODB
  DEFAULT CHARSET = utf8;

插入数据：

INSERT INTO tb_score1(userid, cn_score, math_score, en_score, po_score)
VALUES ('001', 90, 92, 80, 0);
INSERT INTO tb_score1(userid, cn_score, math_score, en_score, po_score)
VALUES ('002', 88, 90, 75.5, 0);
INSERT INTO tb_score1(userid, cn_score, math_score, en_score, po_score)
VALUES ('003', 70, 85, 90, 82);

查询数据表中的内容（即转换前的结果）

								SELECT * FROM tb_score1

转换后：

本质是将userid的每个科目分数分散成一条记录显示出来。

直接上SQL：

SELECT userid, '语文' AS course, cn_score AS score
FROM tb_score1
UNION ALL
SELECT userid, '数学' AS course, math_score AS score
FROM tb_score1
UNION ALL
SELECT userid, '英语' AS course, en_score AS score
FROM tb_score1
UNION ALL
SELECT userid, '政治' AS course, po_score AS score
FROM tb_score1
ORDER BY userid

这里将每个userid对应的多个科目的成绩查出来，通过UNION ALL将结果集加起来，达到上图的效果。

附：UNION与UNION ALL的区别：

1.对重复结果的处理：UNION会去掉重复记录，UNION ALL不会；

2.对排序的处理：UNION会排序，UNION ALL只是简单地将两个结果集合并；

3.效率方面的区别：因为UNION 会做去重和排序处理，因此效率比UNION ALL慢很多；