How to select number of days from history table? - sql

Question

EDITED: history table may contain rows that have no effect in the time calculation (such as comments, or sending reminders...)
I have 2 tables with the following structure
PS: for the sake of simplicity, I avoided going into too much details (in reality, I have another table for STATUS codes, and another for users, etc.)
Tickets Table
--------------------------------------------------
| ID | Ticket_Details | Issued_By | Status |
--------------------------------------------------
| 001 | 'PC not working' | 'John' | On Hold |
| 002 | 'Printer broken' | 'Mike' | Rejected |
| 003 | 'Network down' | 'Alex' | Submitted |
| .. | ... | .. | ... |
--------------------------------------------------
History Table
------------------------------------------------------------------------
| ID | Ticket_ID | Ticket_Status | History_Details | Insert_Date |
------------------------------------------------------------------------
| 01 | 001 | new | submitted | 23-Feb-2015 |
| 02 | 001 | submitted | assigned to [Frank] | 25-Feb-2015 |
| 03 | 001 | submitted | commented 'needs ti.'| 25-Feb-2015 |
| 04 | 001 | assigned | put on hold by[Frank]| 26-Feb-2015 |
| 05 | 001 | on hold | reminder sent | 01-Mar-2015 |
| 06 | 002 | new | submitted | 23-Feb-2015 |
| 07 | 002 | submitted | rejected by [Sam] | 24-Feb-2015 |
| 08 | 003 | new | submitted | 25-Feb-2015 |
------------------------------------------------------------------------
Each row of the history table contains the following information:
1- the id (auto inc) primary key
2- a foreign key to ticket id
3- the status of the ticket before it was modified
4- remarks of what action was done to the ticket
5- the date [and time] of the action
Notice, that some rows describe a change in the status, while some rows, there was no change. Only a comment or a reminder was sent, while the ticket stayed at its status.
now what I want to achieve is a table that shows the current status of the ticket with the number of days it had been assigned to this status. So for the sample data above, the output should be: (considering today's date is 1-Mar-2015)
desired output
--------------------------------------------------
| ID | Ticket_Details | Status | Since |
--------------------------------------------------
| 001 | 'PC not working' | On Hold | 3 days |
| 002 | 'Printer broken' | Rejeced | 5 days |
| 003 | 'Network down' | submitted | 4 days |
| .. | ... | .. | |
--------------------------------------------------
Basically, I am using the information stored in insert_date in history table to determine how long the ticket has been in that status.
Here is what I tried:
select
t.id,
t.ticket_details,
t.status,
datediff(day, h.insert_date, getdate() ) "since"
from
tickets t
left join history h on t.id = h.ticket_id
and h.id = ( select max(id) from history h2
where h2.ticket_id = h.ticket_id
AND h2.ticket_status = t.status )
I am telling SQL to look for the last time in history where this ticket has had this status..
but I got in-accurate data since some of the "since" values came out nulls.
What mistake am I doing?

Answer 1

I think this is a good use of cross apply:
select t.*, datediff(day, minid, getdate()) as days_since
from tickets t outer apply
(select min(insert_date) as minid
from history h
where h.ticket_id = t.ticket_id and h.ticket_status = t.status
) h;
Note: this returns the earliest date that a ticket was at the current status. This assumes that tickets never return to a previous status, which is consistent with the information in your question.

Answer 2

Maybe try
select
t.id,
t.ticket_details,
t.status,
datediff(day, h.insert_date, getdate() ) "since"
from
tickets t
left join (SELECT MAX(insert_date) as insert_date,ticket_id
FROM history group by ticket_id) as h
on t.id=h.ticket_id