题35：

（1）首先通过子查询，将两个表拼接，过滤无用的数据；

select 'succeeded' as 'state',success_date as 'dt'
from Succeeded
where success_date between ' 2019-01-01' and '2019-12-31 '
union 
select 'failed'as 'state',fail_date as 'dt'
from Failed
where fail_date between ' 2019-01-01' and '2019-12-31 '

（2）开窗作差，得到标记:subdate (date, INTERVAL expr unit)，此函数有三个参数：

• 日期：第一个指定的日期；
• expr：要减去的时间日期间隔的值。

（3）根据标记分组，分别拿到这组的极值就是开始和结束日期。

select state as period_state,min(dt ) as start_date,max(dt) as end_date
from
(
    select *,subdate(dt,rank()over(partition by state order by  dt)) as dif
    from 
    (
        select 'succeeded' as 'state',success_date as 'dt'
        from Succeeded
        where success_date between ' 2019-01-01' and '2019-12-31 '
        union 
        select 'failed'as 'state',fail_date as 'dt'
        from Failed
        where fail_date between ' 2019-01-01' and '2019-12-31 '
    )t1
)t2
group by state,dif
order by dt;