https://leetcode-cn.com/problems/report-contiguous-dates/
Table: Failed
±-------------±--------+
| Column Name | Type |
±-------------±--------+
| fail_date | date |
±-------------±--------+
该表主键为 fail_date。
该表包含失败任务的天数.
Table: Succeeded
±-------------±--------+
| Column Name | Type |
±-------------±--------+
| success_date | date |
±-------------±--------+
该表主键为 success_date。
该表包含成功任务的天数.
系统 每天 运行一个任务。每个任务都独立于先前的任务。任务的状态可以是失败或是成功。
编写一个 SQL 查询 2019-01-01 到 2019-12-31 期间任务连续同状态 period_state 的起止日期（start_date 和 end_date）。即如果任务失败了，就是失败状态的起止日期，如果任务成功了，就是成功状态的起止日期。
最后结果按照起始日期 start_date 排序
查询结果样例如下所示:
Failed table:
±------------------+
| fail_date |
±------------------+
| 2018-12-28 |
| 2018-12-29 |
| 2019-01-04 |
| 2019-01-05 |
±------------------+
Succeeded table:
±------------------+
| success_date |
±------------------+
| 2018-12-30 |
| 2018-12-31 |
| 2019-01-01 |
| 2019-01-02 |
| 2019-01-03 |
| 2019-01-06 |
±------------------+
Result table:
±-------------±-------------±-------------+
| period_state | start date | end date |
±-------------±-------------±-------------+
| succeeded | 2019-01-01 | 2019-01-03 |
| failed | 2019-01-04 | 2019-01-05 |
| succeeded | 2019-01-06 | 2019-01-06 |
±-------------±-------------±-------------+
结果忽略了 2018 年的记录，因为我们只关心从 2019-01-01 到 2019-12-31 的记录
从 2019-01-01 到 2019-01-03 所有任务成功，系统状态为 “succeeded”。
从 2019-01-04 到 2019-01-05 所有任务失败，系统状态为 “failed”。
从 2019-01-06 到 2019-01-06 所有任务成功，系统状态为 “succeeded”。
思路：
看到这道题，就有一个想法，我可以将每天的数据查询出来，然后进行排序，把状态相同的数据进行合并，找到开始和结束时间
不过遇到了难点，就是怎么合并数据，我的想法是从第一条开始，一条一条做标记，如果状态变化，标记就修改，这样同一种标记的就可以合并
不过，还是不知道怎么做
最后看了题解，可以使用自定义的变量
set @prev = null;
set @tmp = 0;
但是不能直接这么用，因为leetcode只认语句sql，如果像上面这么set，得多条sql
然后还是题解,把设置变量的代码写到查询中，组成一个关联表
答案：
select period_state ,min(date) start_date,max(date) end_date
from
(select date,period_state,
if(@prev = period_state ,@tmp := @tmp, @tmp := @tmp + 1) as rank,
@prev := period_state as prev
from
(select * from (
select ‘succeeded’ period_state,success_date date from Succeeded where success_date between ‘2019-01-01’ and ‘2019-12-31’
union all
select ‘failed’ period_state,fail_date date from Failed where fail_date between ‘2019-01-01’ and ‘2019-12-31’) t0
order by date
)t1,(select @prev := null,@tmp := 0) as init)t2
group by rank
解剖：
//查询每一天数据，按时间排序
select * from (
select ‘succeeded’ period_state,success_date date from Succeeded where success_date between ‘2019-01-01’ and ‘2019-12-31’
union all
select ‘failed’ period_state,fail_date date from Failed where fail_date between ‘2019-01-01’ and ‘2019-12-31’) t0
order by date
//将查出来的数据，进行标记
select date,period_state,
if(@prev = period_state ,@tmp := @tmp, @tmp := @tmp + 1) as rank,
@prev := period_state as prev
from
(select * from (
select ‘succeeded’ period_state,success_date date from Succeeded where success_date between ‘2019-01-01’ and ‘2019-12-31’
union all
select ‘failed’ period_state,fail_date date from Failed where fail_date between ‘2019-01-01’ and ‘2019-12-31’) t0
order by date
)t1,(select @prev := null,@tmp := 0) as init
//通过标记，进行分组
select period_state ,min(date) start_date,max(date) end_date
from
(select date,period_state,
if(@prev = period_state ,@tmp := @tmp, @tmp := @tmp + 1) as rank,
@prev := period_state as prev
from
(select * from (
select ‘succeeded’ period_state,success_date date from Succeeded where success_date between ‘2019-01-01’ and ‘2019-12-31’
union all
select ‘failed’ period_state,fail_date date from Failed where fail_date between ‘2019-01-01’ and ‘2019-12-31’) t0
order by date
)t1,(select @prev := null,@tmp := 0) as init)t2
group by rank
