SQL Server query to return difference between dates in two rows and initial status column in OUTPUT

Question

I have two tables, current_perf_rat and history_perf_rat

• current_perf_rat with columns PersonRatingID, FirstName, LastName, Email, CurrentStatusID
• history_perf_rat with columns PersonRatingID, StatusID, StatusName, StatusTimestamp, isCurrent

current_perf_rat

PersonRatingID	FirstName	LastName	Email	CurrentStatusID
123	XZY	HUG	xyz@gmail.com	678
456	smith	john	smith @xyz.com	746

history_perf_rat:

PersonRatingID	StatusID	StatusName	StatusTimestamp	isCurrent
123	678	Rating Given	2021-10-01 10:00	Y
123	746	Rating in Draft	2021-09-28 15:00	N
123	567	Started Rating	2021-09-01 02:00	N
456	678	Rating Given	2021-08-13 22:00	Y
456	746	Rating in Draft	2021-09-17 15:00	N
456	567	Started Rating	2021-08-01 02:00	N
456	561	Initiated Rating	2020-08-01 02:00	N

I want to create a query to find out how much time does it take for an application to move from the very first status to next status for all the Person Rating Ids in year 2021 only.

This below query gives me the row details and their row num -

SELECT DISTINCT
    current_perf_rat.PersonRatingID,
    history_perf_rat.StatusName,
    CAST(history_perf_rat.StatusTimestamp AS DATE) Application_date,
    ROW_NUMBER() OVER (ORDER BY history_perf_rat.StatusTimestamp ASC) rn
FROM 
    current_perf_rat, history_perf_rat
WHERE 
    current_perf_rat.PersonRatingID  = history_perf_rat .PersonRatingID 
    --and current_perf_rat.PersonRatingID =123 -- only one rating
ORDER BY
    history_perf_rat.StatusTimestamp ASC

This gives me the output I want to work with -

PersonRatingID	StatusName	Application_date	rn
456	Started Rating	2021-08-01	2
456	Initiated Rating	2020-08-01	1
123	Rating in Draft	2021-09-28	2
123	Started Rating	2021-09-01	1

Expected output:

PersonRatingID	StatusName	Time_Taken
456	Initiated Rating	1 Year
123	Started Rating	1 year 28 days

• How to modify this SQL statement to get the difference between these two dates and the initial status name ie "Initiated Rating" and started rating ? Is there a way to do it by using LAG or something?

• Also, How to add a condition(Parameter) so that I Can chose the year for which this should be done ?

For example, in the above result for 456, 2020 timestamp is also included above but I only want to include 2021. Can this me done dynamically ?

Answer 1

You can use ROW_NUMBER to only get the first row, and LEAD to get the next Timestamp after the first

SELECT
  h.PersonRatingID,
  h.StatusName,
  Time_Taken = DATEDIFF(day, StatusTimestamp, NextTime)
FROM (
    SELECT *,
      NextTime = LEAD(h.StatusTimestamp)
                        OVER (PARTITION BY h.PersonRatingID ORDER BY h.StatusTimestamp),
      rn = ROW_NUMBER() OVER (PARTITION BY h.PersonRatingID ORDER BY h.StatusTimestamp)
    FROM history_perf_rat h
    WHERE h.StatusTimestamp >= '2021-01-01'
) h
WHERE h.rn = 1;

db<>fiddle