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部门表 Department：

Column Name	Type
id	int
revenue	int
month	varchar

(id, month) 是表的联合主键。

这个表格有关于每个部门每月收入的信息。 月份（month）可以取下列值 [“Jan”,“Feb”,“Mar”,“Apr”,“May”,“Jun”,“Jul”,“Aug”,“Sep”,“Oct”,“Nov”,“Dec”]。

问题

编写一个 SQL 查询来重新格式化表，使得新的表中有一个部门 id 列和一些对应 每个月 的收入（revenue）列。

示例

Department 表：

id	revenue	month
1	8000	Jan
2	9000	Jan
3	10000	Feb
1	7000	Feb
1	6000	Mar

结果表：

id	Jan_Revenue	Feb_Revenue	Mar_Revenue	…	Dec_Revenue
1	8000	7000	6000	…	null
2	9000	null	null	…	null
3	null	10000	null	…	null

注意，结果表有 13 列 (1个部门 id 列 + 12个月份的收入列)。

来源：力扣（LeetCode）

链接：https://leetcode-cn.com/problems/reformat-department-table 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

解答

第一步：列拆成行

需要将revenue列根据month拆成行，有两种方式，CASE WHEN或者IF语句。

第二步：行合并

根据题意，需要根据id，将相同id的行进行合并，结果如下表，采用分组的方式，可以对各月收入列求SUM/MAX/MIN/AVG都可以。

法一:Case When

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

法二：IF

select distinct id,    sum(IF(month="Jan",revenue,null)) as Jan_Revenue,    sum(IF(month="Feb",revenue,null)) as Feb_Revenue,    sum(IF(month="Mar",revenue,null)) as Mar_Revenue,    sum(IF(month="Apr",revenue,null)) as Apr_Revenue,    sum(IF(month="May",revenue,null)) as May_Revenue,    sum(IF(month="Jun",revenue,null)) as Jun_Revenue,    sum(IF(month="Jul",revenue,null)) as Jul_Revenue,    sum(IF(month="Aug",revenue,null)) as Aug_Revenue,    sum(IF(month="Sep",revenue,null)) as Sep_Revenue,    sum(IF(month="Oct",revenue,null)) as Oct_Revenue,    sum(IF(month="Nov",revenue,null)) as Nov_Revenue,    sum(IF(month="Dec",revenue,null)) as Dec_Revenuefrom Department group by id order by id;