MySQL : Select number of unique days between two dates - mysql

Question

I have a table that shows events that took place during several days in a spatial grid and I want to select the number of unique days for each cell of the grid in order to obtain the number of days where an event happend, here my table structure :
+-----+------------+------------+---------+---------+
| id | start_date | end_date | id_cell | event |
+-----+------------+------------+---------+---------+
| 1 | 2017-03-01 | 2017-03-04 | 250 | envent1 |
| 2 | 2017-03-01 | 2017-03-04 | 251 | envent1 |
| 3 | 2017-03-01 | 2017-03-04 | 307 | envent1 |
| 4 | 2017-03-01 | 2017-03-04 | 308 | envent1 |
| 5 | 2017-03-01 | 2017-03-09 | 250 | event2 |
| 9 | 2017-02-24 | 2017-03-03 | 250 | event3 |
| 13 | 2017-02-24 | 2017-03-24 | 250 | event4 |
| 17 | 2017-02-24 | 2017-03-02 | 250 | event5 |
| 21 | 2017-01-04 | 2017-01-25 | 250 | event6 |
| 25 | 2017-03-26 | 2017-03-28 | 250 | event2 |
+-----+------------+------------+---------+---------+
For example, the expected result for the cell with 250 as id is 51 days :
2017-01-04 -> 2017-01-25 = 21 days
2017-02-24 -> 2017-03-24 = 28 days
2017-03-26 -> 2017-03-28 = 2 days
The other dates are all included between 2017-02-24 and 2017-03-24 so they don't have to be counted so 21 + 28 + 2 = 51 days.
I tried to use DATEDIFF() like this :
select datediff(max(end_date) , min(start_date) ) from cell_date where id_cell = 250
The result is 83 because it counts the number of days between 2017-01-25 and 2017-03-01, the days where no event happened.
I tried some requests with DATEDIFF but I couldn't figure out how to do it. Someone can help me please ? Thanks in advance.

Answer 1

You can achieve this by grouping by cell_id and calculating the sum of the individual differences:
Select cell_id,[other columns],Sum(datediff(days,start_date,end_date)) as Days
From my_table
group by cell_id,[other columns]
Edit:
For your need I think you should use an intermediate table to store individual days(sure it's not the best way to do it), join them with your events and then select the distinct days in your result. Here is the code to achieve it
/*Your example Talbe*/
DECLARE #T
TABLE(ID INT,startDate DATE,EndDate DATE,id_cell INT,evnt NVARCHAR(20) )
INSERT INTO #T
VALUES
(1,'2017-03-01','2017-03-04',250,'event1'),
(2,'2017-03-01','2017-03-04',251,'event1'),
(3,'2017-03-01','2017-03-04',307,'event1'),
(4,'2017-03-01','2017-03-04',308,'event1'),
(5,'2017-03-01','2017-03-09',250,'event2'),
(9,'2017-02-24','2017-03-03',250,'event3'),
(13,'2017-02-24','2017-03-24',250,'event4'),
(17,'2017-02-24','2017-03-02',250,'event5'),
(21,'2017-01-04','2017-01-25',250,'event6'),
(25,'2017-03-26','2017-03-28',250,'event2')
/*Table to store days: ideally get the start and end dates from your table */
DECLARE #STARTDATE DATE='2017-01-04'
DECLARE #ENDDATE DATE='2017-03-28'
DECLARE #DAYS
TABLE(oneday DATE)
WHILE #StartDate <= #endDate
BEGIN
INSERT INTO #days
(
oneday
)
SELECT
#StartDate
SET #StartDate = DATEADD(dd, 1, #StartDate)
END
/*The request */
SELECT id_cell,COUNT(DISTINCT oneday) NUMBER_OF_DAYS
FROM #T t
JOIN #DAYS d ON d.oneday>=t.startDate AND d.oneday<t.EndDate
WHERE id_cell=250
GROUP BY id_cell

Answer 2

You could use max and min date and datediff for obtain the diff in days between the date eg: for event
select event, datediff(max(end_date) , min(star_date) )
from my_table
group by event
or yuo can sum by cell_id for the total for cell_id
select cell_id, sum(datediff(end_date , star_date) )as days
from my_table
group by cell_id
or
Looking to your sample seems you need max end_date for some event occurring starting from the same date
select id_cell, sum(datediff(max_end_date, start_date)) as days
from (
select id_cell, start_date, max(end_date) as max_end_date
from cell_date
group by id_cell, start_date ) t
group by id_cell