行转列
**1179. 重新格式化部门表** （难度：<font color=green>**简单**</font>）
部门表 Department：
```language
+---------------+---------+
| 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"]。
```
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。
查询结果格式如下面的示例所示：
```language
Department 表：
+------+---------+-------+
| id | revenue | month |
+------+---------+-------+
| 1 | 8000 | Jan |
| 2 | 9000 | Jan |
| 3 | 10000 | Feb |
| 1 | 7000 | Feb |
| 1 | 6000 | Mar |
+------+---------+-------+
查询得到的结果表：
+------+-------------+-------------+-------------+-----+-------------+
| 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)。
```
**解法：**
```sql
select id,
max(if(month='Jan' ,revenue,null)) as 'Jan_Revenue',
max(if(month='Feb' ,revenue,null)) as 'Feb_Revenue',
max(if(month='Mar' ,revenue,null)) as 'Mar_Revenue',
max(if(month='Apr' ,revenue,null)) as 'Apr_Revenue',
max(if(month='May' ,revenue,null)) as 'May_Revenue',
max(if(month='Jun' ,revenue,null)) as 'Jun_Revenue',
max(if(month='Jul' ,revenue,null)) as 'Jul_Revenue',
max(if(month='Aug' ,revenue,null)) as 'Aug_Revenue',
max(if(month='Sep' ,revenue,null)) as 'Sep_Revenue',
max(if(month='Oct' ,revenue,null)) as 'Oct_Revenue',
max(if(month='Nov' ,revenue,null)) as 'Nov_Revenue',
max(if(month='Dec' ,revenue,null)) as 'Dec_Revenue'
from Department
group by id
```