LeetCode1179. 重新格式化部门表，使用group by与case聚合处理

解题思路

聚合函数本质上就是常见编程语言里的reduce，也就是把一组数据处理成一个单一数据，所以可以这样写：

select id, sum(revenue) as total
from Department
group by id

来表示一个部门的总收入。

这个操作在javascript中类似于这样：

departments.reduce(total, department => total + department.revenue)

max等其它聚合函数同理，都可以找到用reduce表示的等价形式。

那么代码

sum(case month when 'Jan' then revenue end) as Jan_Revenue,

的意思就是说把所有的revenue聚合处理，处理的方法是，如果month的值是Jan，那么结果就是revenue，否则忽略。

代码

select id,
    sum(case month when 'Jan' then revenue end) as Jan_Revenue,
    sum(case month when 'Feb' then revenue end) as Feb_Revenue,
    sum(case month when 'Mar' then revenue end) as Mar_Revenue,
    sum(case month when 'Apr' then revenue end) as Apr_Revenue,
    sum(case month when 'May' then revenue end) as May_Revenue,
    sum(case month when 'Jun' then revenue end) as Jun_Revenue,
    sum(case month when 'Jul' then revenue end) as Jul_Revenue,
    sum(case month when 'Aug' then revenue end) as Aug_Revenue,
    sum(case month when 'Sep' then revenue end) as Sep_Revenue,
    sum(case month when 'Oct' then revenue end) as Oct_Revenue,
    sum(case month when 'Nov' then revenue end) as Nov_Revenue,
    sum(case month when 'Dec' then revenue end) as Dec_Revenue
from Department
group by id