一、行转列
1、使用case…when…then
2、使用SUM(IF()) 生成列
3、使用SUM(IF()) 生成列 + WITH ROLLUP 生成汇总行
4、使用SUM(IF()) 生成列，直接生成汇总结果，不再利用子查询
5、使用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
6、动态查询列值不确定的情况
7、合并字段显示：group_concat()
二、列转行
————————————————
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。
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
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。
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：
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：
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：
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：
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。
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。
提示：请勿发布广告垃圾评论，否则封号处理！！