静态行转列
- 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
- 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
（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。
- 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;
with rollup是用来在分组统计数据的基础上再进行统计汇总，即用来得到group by的汇总信息；
IFNULL(userid,'TOTAL') AS userid 则是在with rollup上汇总那行数据上，userid列为null时，命名TOTAL
动态
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;
