【leetcode】1179. Reformat Department Table

题目如下：

SQL Schema

Table: Department

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| id            | int     |
| revenue       | int     |
| month         | varchar |
+---------------+---------+
(id, month) is the primary key of this table.
The table has information about the revenue of each department per month.
The month has values in ["Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"].

Write an SQL query to reformat the table such that there is a department id column and a revenue column for each month.

The query result format is in the following example:

Department table:
+------+---------+-------+
| id   | revenue | month |
+------+---------+-------+
| 1    | 8000    | Jan   |
| 2    | 9000    | Jan   |
| 3    | 10000   | Feb   |
| 1    | 7000    | Feb   |
| 1    | 6000    | Mar   |
+------+---------+-------+

Result table:
+------+-------------+-------------+-------------+-----+-------------+
| id   | Jan_Revenue | Feb_Revenue | Mar_Revenue | ... | Dec_Revenue |
+------+-------------+-------------+-------------+-----+-------------+
| 1    | 8000        | 7000        | 6000        | ... | null        |
| 2    | 9000        | null        | null        | ... | null        |
| 3    | null        | 10000       | null        | ... | null        |
+------+-------------+-------------+-------------+-----+-------------+

Note that the result table has 13 columns (1 for the department id + 12 for the months).

解题思路：难得有免费的SQL题目。我的SQL性能比较低，就是做12次左连接，求出每个月的revenue。

代码如下：

# Write your MySQL query statement below
select d0.id, Jan_Revenue,Feb_Revenue,Mar_Revenue,Apr_Revenue,May_Revenue,Jun_Revenue,Jul_Revenue,
Aug_Revenue,Sep_Revenue,Oct_Revenue,Nov_Revenue,Dec_Revenue from
(select distinct  id from Department order by id ) d0
left join
(select id,revenue as Jan_Revenue from Department where month = 'Jan') d1 on d0.id = d1.id
left join
(select id,revenue as Feb_Revenue from Department where month = 'Feb') d2 on d0.id = d2.id
left join
(select id,revenue as Mar_Revenue from Department where month = 'Mar') d3 on d0.id = d3.id
left join
(select id,revenue as Apr_Revenue from Department where month = 'Apr') d4 on d0.id = d4.id
left join
(select id,revenue as May_Revenue from Department where month = 'May') d5 on d0.id = d5.id
left join
(select id,revenue as Jun_Revenue from Department where month = 'Jun') d6 on d0.id = d6.id
left join
(select id,revenue as Jul_Revenue from Department where month = 'Jul') d7 on d0.id = d7.id
left join
(select id,revenue as Aug_Revenue from Department where month = 'Aug') d8 on d0.id = d8.id
left join
(select id,revenue as Sep_Revenue from Department where month = 'Sep') d9 on d0.id = d9.id
left join
(select id,revenue as Oct_Revenue from Department where month = 'Oct') d10 on d0.id = d10.id
left join
(select id,revenue as Nov_Revenue from Department where month = 'Nov') d11 on d0.id = d11.id
left join
(select id,revenue as Dec_Revenue from Department where month = 'Dec') d12 on d0.id = d12.id
order by id;