Calculating Days Between Dates in Separate Rows For Same UnitID - sql-server

Question

I am trying to calculate the time a commercial real estate space sits vacant. I have move-in & move-out dates for each tenant that has occupied that unit. It is easy to calculate the occupied time of each tenant as that data is within the same row. However, I want to calculate the vacant time: the time between move-out of the previous tenant and move-in of the next tenant. These dates appear in separate rows.
Here is a sample of what I have currently:
SELECT
uni_vch_UnitNo AS UnitNumber,
uty_vch_Code AS UnitCode,
uty_int_Id AS UnitID, tul_int_FacilityId AS FacilityID,
tul_dtm_MoveInDate AS Move_In_Date,
tul_dtm_MoveOutDate AS Move_Out_Date,
DATEDIFF(day, tul_dtm_MoveInDate, tul_dtm_MoveOutDate) AS Occupancy_Days
FROM TenantUnitLeases
JOIN units
ON tul_int_UnitId = uni_int_UnitId
JOIN UnitTypes
ON uni_int_UnitTypeId = uty_int_Id
WHERE
tul_int_UnitId = '26490'
ORDER BY tul_dtm_MoveInDate ASC
Is there a way to assign an id to each row in chronological, sequential order and find the difference between row 2 move-in date less row 1 move-out date and so on?
Thank you in advance for the help.

Answer 1

I can't really tell which tables provide which columns for your query. Please alias and dot-qualify them in the future.
If you're using SQL 2012 or later, you've got LEAD and LAG functions which do exactly what you want: bring a "leading" or "lagging" row into a current row. See if this works (hopefully it should at least get you started):
SELECT
uni_vch_UnitNo AS UnitNumber,
uty_vch_Code AS UnitCode,
uty_int_Id AS UnitID, tul_int_FacilityId AS FacilityID,
tul_dtm_MoveInDate AS Move_In_Date,
tul_dtm_MoveOutDate AS Move_Out_Date,
DATEDIFF(day, tul_dtm_MoveInDate, tul_dtm_MoveOutDate) AS Occupancy_Days
, LAG(tul_dtm_MoveOutDate) over (partition by uni_vch_UnitNo order by tul_dtm_MoveOutDate) as Previous_Move_Out_Date
, DATEDIFF(day,LAG(tul_dtm_MoveOutDate) over (partition by uni_vch_UnitNo order by tul_dtm_MoveOutDate),tul_dtm_MoveInDate) as Days_Vacant
FROM TenantUnitLeases
JOIN units
ON tul_int_UnitId = uni_int_UnitId
JOIN UnitTypes
ON uni_int_UnitTypeId = uty_int_Id
WHERE
tul_int_UnitId = '26490'
ORDER BY tul_dtm_MoveInDate ASC

Answer 2

Just comparing a value from the current row with a value in the previous row is functionality provided by the lag() function.
Try this in your query:
select...
tul_dtm_MoveInDate AS Move_In_Date,
tul_dtm_MoveOutDate AS Move_Out_Date,
DateDiff(day, Lag(tul_dtm_MoveOutDate,1) over(partition by uty_vch_Code, tul_int_FacilityId order by tul_dtm_MoveInDate), tul_dtm_MoveInDate) DaysVacant,
...

Answer 3

This needs a window function or correlated sub query. The goal is to provide the previous move out date for each row, which is in turn a function of that row. The term 'window' in this context means to apply an aggregate function over a smaller range than the whole set.
If you had a function called GetPreviousMoveOutDate, the parameters would be the key to filter on, and the ranges to search within the filter. So we would pass the UnitID as the key and the MoveInDate for this row, and the function should return the most recent MoveOutDate for the same unit that is before the passed in date. By getting the max date before this one, we will ensure we get only the previous occupancy if it exists.
To use a sub-query in ANSI-SQL you just add the select as a column. This should work on MS-SQL as well as other DB platforms; however, it requires using aliases for the table names so they can be referenced in the query more than once. I've updated your sample SQL with aliases using the AS syntax, although it looks redundant to your table naming convention. I added a uni_dtm_UnitFirstAvailableDate to your units table to handle the first vacancy, but this can be a default:
SELECT
uni.uni_vch_UnitNo AS UnitNumber,
uty.uty_vch_Code AS UnitCode,
uty.uty_int_Id AS UnitID, tul_int_FacilityId AS FacilityID,
tul.tul_dtm_MoveInDate AS Move_In_Date,
tul.tul_dtm_MoveOutDate AS Move_Out_Date,
DATEDIFF(day, tul.tul_dtm_MoveInDate, tul.tul_dtm_MoveOutDate) AS Occupancy_Days,
-- select the date:
(SELECT MAX (prev_tul.tul_dtm_MoveOutDate )
FROM TenantUnitLeases AS prev_tul
WHERE prev_tul.tul_int_UnitId = tul.tul_int_UnitId
AND prev_tul.tul_dtm_MoveOutDate > tul.tul_dtm_MoveInDate
AND prev_tul.tul_dtm_MoveOutDate is not null
) AS previous_moveout,
-- use the date in a function:
DATEDIFF(day, tul.tul_dtm_MoveInDate,
ISNULL(
(SELECT MAX (prev_tul.tul_dtm_MoveOutDate )
FROM TenantUnitLeases AS prev_tul
WHERE prev_tul.tul_int_UnitId = tul.tul_int_UnitId
AND prev_tul.tul_dtm_MoveOutDate > tul.tul_dtm_MoveInDate
AND prev_tul.tul_dtm_MoveOutDate is not null
) , uni.uni_dtm_UnitFirstAvailableDate) -- handle first occupancy
) AS Vacancy_Days
FROM TenantUnitLeases AS tul
JOIN units AS uni
ON tul.tul_int_UnitId = uni.uni_int_UnitId
JOIN UnitTypes AS uty
ON uni.uni_int_UnitTypeId = uty.uty_int_Id
WHERE
tul.tul_int_UnitId = '26490'
ORDER BY tul.tul_dtm_MoveInDate ASC