题解 | #行列互换#

比较简单的换行，就是把年的数据合并起来了，再加上判断是几月的数据，一相加即可.聚合+判断
方法一:group by +sum()+case when 做法
select 
年,
sum(case when 月=1 then 值 else 0 end) as m1,
sum(case when 月=2 then 值 else 0 end) as m2,
sum(case when 月=3 then 值 else 0 end) as m3,
sum(case when 月=4 then 值 else 0 end) as m4
from cook
group by 年
order by 年

方法二:group by +sum()+if()
select 
年,
sum(if(月=1,值,0)) as m1,
sum(if(月=2,值,0)) as m2,
sum(if(月=3,值,0)) as m3,
sum(if(月=4,值,0)) as m4
from cook
group by 年
order by 年