Query to select intervals from a table

Question

I have a table that defines time intervals.

 _______________________________
| id | description | start_date |
|____|_____________|____________|
|  1 | First       |    NULL    |
|  3 | Third       | 2009-12-18 |
|  5 | Second      | 2009-10-02 |
|  4 | Last        | 2009-12-31 |
|____|_____________|____________|

It stores only the start date, the end date is a day before next date that follows.

I would like to have the next result:

 ____________________________________________
| id | description | start_date |  end_date  |
|____|_____________|____________|____________|
|  1 | First       |    NULL    | 2009-10-01 |
|  5 | Second      | 2009-10-02 | 2009-12-17 |
|  3 | Third       | 2009-12-18 | 2009-12-30 |
|  4 | Last        | 2009-12-31 |    NULL    |
|____|_____________|____________|____________|

How should I write this query, since a row contains values from other rows?

(I think MySQL function DATE_SUB could be useful.)

Answer 1

if you are using PHP, just calculate the previous date in the script. that's easier.

if not, would something like:

SELECT ID,description,start_date,start_date-1 AS end_date FROM ...

work?

UPDATE: it works partially, as it returns 20091224 for startdate 2009-12-25 but won't work for dates like 2009-12-01 etc.

Answer 2

try

   Select d.Id, d.Description, d.Start_Date, 
       n.Start_Date - 1 EndDate
   From Table d 
     Left Join Table n
        On n.Start_Date = 
              (Select Min(Start_date)
               From Table
               Where Start_Date > Coalesce(d.Start_Date, '1/1/1900')

Answer 3

try

select id, description, start_date, end_date from
  (
    select @rownum_start:=@rownum_start+1 rank, id, description, start_date
    from inter, (select @rownum_start:=0) p
    order by start_date
  ) start_dates
left join
  (
    select @rownum_end:=@rownum_end+1 rank, start_date - interval 1 day as end_date
    from inter, (select @rownum_end:=0) p
    where start_date is not null
    order by start_date
  ) end_dates
using (rank)

where inter is your table

This actually returns:

mysql> select id, description, start_date, end_date from ...
+----+-------------+------------+------------+
| id | description | start_date | end_date   |
+----+-------------+------------+------------+
|  1 | First       | NULL       | 2009-10-01 |
|  5 | Second      | 2009-10-02 | 2009-12-17 |
|  3 | Third       | 2009-12-18 | 2009-12-30 |
|  4 | Last        | 2009-12-31 | NULL       |
+----+-------------+------------+------------+
4 rows in set (0.00 sec)

Answer 4

SELECT d.id, d.description, MIN(d.start_date), MIN(d2.start_date) - INTERVAL 1
DAY AS end_date
FROM start_dates d
LEFT OUTER JOIN start_dates d2 ON ( d2.start_date > d.start_date OR d.start_date IS NULL )
GROUP BY d.id, d.description
ORDER BY d.start_date ASC