计算多个日期范围之间的总天数

Question

I have a problem figuring out how to calculate total days between different date ranges using MySQL.

I need to count total of days between different date ranges without days that include each other date range.

Data example:

from	to
2021/08/28	2021/09/29
2021/08/29	2021/09/01
2021/09/01	2021/09/01

Date ranges example and output

Dates   2021-08-28  2021-08-29  2021-08-30  2021-08-31  2021-09-01  2021-09-02  2021-09-03  2021-09-04 
Range1  |--------------------|         
Range2                                      |--------------------|         
Range3                                                                          |--------------------|         

Total Days: 6

Dates   2021-08-28  2021-08-29  2021-08-30  2021-08-31  2021-09-01  2021-09-02  2021-09-03  2021-09-04 
Range1  |--------------------|         
Range2              |--------------------------------------------|         
Range3                                                  |--------|         

Total Days: 5

Answer 1

Possibly the simplest method is a recursive CTE:

with recursive dates as (
      select `from`, `to`
      from t
      union all
      select `from` + interval 1 day, `to`
      from dates
      where `from` < `to`
     )
select count(distinct `from`)
from dates;

Note that from and to are really bad names for columns because they are SQL keywords.

EDIT:

In MySQL 5.7, you can use a tally table -- a table of numbers.

Assuming your original table has enough rows for the widest time span, you can use:

select count(distinct `from` + interval (n - 1) day)
from t cross join
     (select (@rn := @rn + 1) as n
      from t cross join
           (select @rn := 0) params
     )  n
     on `from` + interval (n - 1) day <= `to`;

If your table is really big, you might want a limit for the widest time period.