SQL - Getting date difference between two rows between three tables

Question

I've scoured the forums, but couldn't quite find a proper solution.

I have two tables with the following information:

-TableA-

   Id   |   Created
  11111 | 2016-01-01
  22222 | 2016-02-02
  33333 | 2016-03-03

-TableB-

   Id   |   Created   |   Comment   
  11111 | 2016-01-01  | Blah Blah Blah
  11111 | 2016-01-02  | Blah Blah Blah
  11111 | 2016-01-15  | Blah Blah Blah
  11111 | 2016-01-17  | Blah Blah Blah
  22222 | 2016-02-02  | Blah Blah Blah
  22222 | 2016-02-05  | Blah Blah Blah
  22222 | 2016-02-09  | Blah Blah Blah
  33333 | 2016-03-03  | Blah Blah Blah
  33333 | 2016-03-14  | Blah Blah Blah

TableA is the master table (it has a whole bunch of other fields, but the important thing is the ID and the Created date field), while TableB is a comment table that gets tied back to TableA.

What I'm trying to do is to calculate the time difference between two rows in TableB and then isolate the very first row where the record is created. I figured the best way to do this would be to use TableA to provide the definitive Created date and somehow use that against TableB after I obtain all of the calculated time differences.

I've written out a reasonable query for TableB to give me the calculated date differences:

SELECT C1.Id,
       C1.Created,
       MIN(C2.Created) AS Created2,
       DATEDIFF(C1.Created, MIN(C2.Created) AS DaysDiff
       FROM TableB C1
            LEFT JOIN TableB C2
            ON C1.Id = C2.Id
            AND C2.Created > C1.Created
       GROUP BY C1.Id, C1.Created

-TableB Queried-

   Id   |   Created   |  Created2   |    DaysDiff
  11111 | 2016-01-01  | 2016-01-02  |       1
  11111 | 2016-01-02  | 2016-01-15  |      13
  11111 | 2016-01-15  | 2016-01-17  |       2
  11111 | 2016-01-17  |             |       
  22222 | 2016-02-02  | 2016-02-05  |       3
  22222 | 2016-02-05  | 2016-02-09  |       4
  22222 | 2016-02-09  |             |        
  33333 | 2016-03-03  | 2016-03-14  |      11
  33333 | 2016-03-14  |             |        

But I need to take this one step further and only get the earliest Created record, so it looks like this:

   Id   |   Created   |  Created2   |    DaysDiff
  11111 | 2016-01-01  | 2016-01-02  |       1
  22222 | 2016-02-02  | 2016-02-05  |       3
  33333 | 2016-03-03  | 2016-03-14  |      11

I'm pretty sure I need to do one more JOIN here, but any JOIN I've done usually ends up where I get no records or I get just the Id and Created columns and nothing else.

Thanks for the help!

Answer 1

To get your expected result, you should use this:

SELECT C1.Id,
       C1.Created,
       MIN(C2.Created) AS Created2,
       DATEDIFF(C1.Created, MIN(C2.Created)) AS DaysDiff
FROM (select id, min(created) created from TableB group by id) C1
    JOIN TableB C2
    ON C1.Id = C2.Id
    AND C2.Created > C1.Created
GROUP BY C1.Id, C1.Created
;

And in the beginning, i thought that the temp C1 table should be tableA, not subquery select min(created) from tableB. If that's true then change that line: FROM (....) C1 to From tableA C1