题目链接：https://leetcode-cn.com/problems/reformat-department-table/
题目
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"]。
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：
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)。
来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/reformat-department-table
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解答
第一感觉就是Python或者R中的长宽表转换。
R中可以使用Reshape2包中的dcast函数进行处理。
SQL中考虑用case when进行行列转换。
本地oracle环境进行测试。
-- 创建中间表
create table temp_cwh_department
(
id int,
revenue int,
month varchar2(10)
)
-- 插入测试数据
insert into temp_cwh_department values(1, 8000, 'Jan');
insert into temp_cwh_department values(2, 9000, 'Jan');
insert into temp_cwh_department values(3, 10000, 'Feb');
insert into temp_cwh_department values(1, 7000, 'Feb');
insert into temp_cwh_department values(1, 6000, 'Mar');
-- 查询
select * from temp_cwh_department
-- 测试
select id,
max(case when month = 'Jan' then revenue else null end) as Jan_Revenue,
max(case when month = 'Feb' then revenue else null end) as Feb_Revenue,
max(case when month = 'Mar' then revenue else null end) as Mar_Revenue,
max(case when month = 'Apr' then revenue else null end) as Apr_Revenue,
max(case when month = 'May' then revenue else null end) as May_Revenue,
max(case when month = 'Jun' then revenue else null end) as Jun_Revenue,
max(case when month = 'Jul' then revenue else null end) as Jul_Revenue,
max(case when month = 'Aug' then revenue else null end) as Aug_Revenue,
max(case when month = 'Sep' then revenue else null end) as Sep_Revenue,
max(case when month = 'Oct' then revenue else null end) as Oct_Revenue,
max(case when month = 'Nov' then revenue else null end) as Nov_Revenue,
max(case when month = 'Dec' then revenue else null end) as Dec_Revenue
from temp_cwh_department
group by id
LeetCode提交
---- oracle ----
/* Write your PL/SQL query statement below */
select id,
max(case when month = 'Jan' then revenue else null end) as Jan_Revenue,
max(case when month = 'Feb' then revenue else null end) as Feb_Revenue,
max(case when month = 'Mar' then revenue else null end) as Mar_Revenue,
max(case when month = 'Apr' then revenue else null end) as Apr_Revenue,
max(case when month = 'May' then revenue else null end) as May_Revenue,
max(case when month = 'Jun' then revenue else null end) as Jun_Revenue,
max(case when month = 'Jul' then revenue else null end) as Jul_Revenue,
max(case when month = 'Aug' then revenue else null end) as Aug_Revenue,
max(case when month = 'Sep' then revenue else null end) as Sep_Revenue,
max(case when month = 'Oct' then revenue else null end) as Oct_Revenue,
max(case when month = 'Nov' then revenue else null end) as Nov_Revenue,
max(case when month = 'Dec' then revenue else null end) as Dec_Revenue
from Department
group by id ---- 887ms看看解答、评论，再学习一番。
改用sum，速度快一点。
---- oracle ----
---- oracle ----
/* Write your PL/SQL query statement below */
select id,
sum(case when month = 'Jan' then revenue else null end) as Jan_Revenue,
sum(case when month = 'Feb' then revenue else null end) as Feb_Revenue,
sum(case when month = 'Mar' then revenue else null end) as Mar_Revenue,
sum(case when month = 'Apr' then revenue else null end) as Apr_Revenue,
sum(case when month = 'May' then revenue else null end) as May_Revenue,
sum(case when month = 'Jun' then revenue else null end) as Jun_Revenue,
sum(case when month = 'Jul' then revenue else null end) as Jul_Revenue,
sum(case when month = 'Aug' then revenue else null end) as Aug_Revenue,
sum(case when month = 'Sep' then revenue else null end) as Sep_Revenue,
sum(case when month = 'Oct' then revenue else null end) as Oct_Revenue,
sum(case when month = 'Nov' then revenue else null end) as Nov_Revenue,
sum(case when month = 'Dec' then revenue else null end) as Dec_Revenue
from Department
group by id ---- 603ms利用decode进行修改，再次提交。
---- oracle ----
/* Write your PL/SQL query statement below */
select id,
sum(decode(month, 'Jan', revenue, null)) as Jan_Revenue,
sum(decode(month, 'Feb', revenue, null)) as Feb_Revenue,
sum(decode(month, 'Mar', revenue, null)) as Mar_Revenue,
sum(decode(month, 'Apr', revenue, null)) as Apr_Revenue,
sum(decode(month, 'May', revenue, null)) as May_Revenue,
sum(decode(month, 'Jun', revenue, null)) as Jun_Revenue,
sum(decode(month, 'Jul', revenue, null)) as Jul_Revenue,
sum(decode(month, 'Aug', revenue, null)) as Aug_Revenue,
sum(decode(month, 'Sep', revenue, null)) as Sep_Revenue,
sum(decode(month, 'Oct', revenue, null)) as Oct_Revenue,
sum(decode(month, 'Nov', revenue, null)) as Nov_Revenue,
sum(decode(month, 'Dec', revenue, null)) as Dec_Revenue
from Department
group by id ---- 947ms记录一个MySQL的用法。
---- MySQL ----
# Write your MySQL query statement below
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`; ---- 229ms思考
通过case when进行判断的时候，例如：case when month = 'Jan' then revenue else null end as Jan_Revenue，会导致每一条记录产生一行，与原来表格的行数一致，因为需要通过group by对id进行分组，但是分组之后一定需要一个聚合函数进行计算，此处就用max即可。
oracle中if/else功能的实现方法
1.标准的SQL规范
-- 单个if
if a = xxx then
......
end if;
-- 多个条件
if a = xxx then
......
else
......
end if;
-- 多个if
if a = xxx then
......
elsif a = xxx then
......
end if;
---- 测试：在oracle中不用用于select中，只能用于存储过程2.decode函数
语法：decode(value, if1, then1, if2, then2, if3, then3,..., else)
解释：如果value等于if1时，decode函数的结果返回then1,...,如果不等于任何一个if值，则返回else。
3.case when
case when a = '1' then 'xxxx'
when a = '2' then 'yyyy'
else
'zzzz'
end as column注意点：
- 以 case 开头，以 end 结尾；
- 分支中 when 后跟条件，then 为显示结果；
- else 为除此之外的默认情况，类似于高级语言程序中 switch case 的 default，可以不加；
- end 后跟别名