How to extrapolate dates in SQL Server to calculate the daily counts? - sql

Question

This is how the data looks like. It's a long table
I need to calculate the number of people employed by day
How to write SQL Server logic to get this result? I treid to create a DATES table and then join, but this caused an error because the table is too big. Do I need a recursive logic?

Answer 1

For future questions, don't post images of data. Instead, use a service like dbfiddle. I'll anyhow add a sketch for an answer, with a better-prepared question you could have gotten a complete answer. Anyhow here it goes:
-- extrema is the least and the greatest date in staff table
with extrema(mn, mx) as (
select least(min(hired),min(retired)) as mn
, greatest(max(hired),max(retired)) as mx
from staff
), calendar (dt) as (
-- we construct a calendar with every date between extreme values
select mn from extrema
union all
select dateadd(day, 1, d)
from calendar
where dt < (select mx from extrema)
)
-- finally we can count the number of employed people for each such date
select dt, count(1)
from calendar c
join staff s
on c.dt between s.hired and s.retired
group by dt;
If you find yourself doing this kind of calculation often, it is a good idea to create a calendar table. You can add other attributes to it such as if it is a day of in the middle of the week etc.
With a constraint as:
CHECK(hired <= retired)
the first part can be simplified to:
with extrema(mn, mx) as (
select min(hired) as mn
, max(retired) as mx
from staff
),

Answer 2

Assuming Current Employees have a NULL retirement date
Declare #Date1 date = '2015-01-01'
Declare #Date2 date = getdate()
Select A.Date
,HeadCount = count(B.name)
From ( Select Top (DateDiff(DAY,#Date1,#Date2)+1)
Date=DateAdd(DAY,-1+Row_Number() Over (Order By (Select Null)),#Date1)
From master..spt_values n1,master..spt_values n2
) A
Left Join YourTable B on A.Date >= B.Hired and A.Date <= coalesce(B.Retired,getdate())
Group BY A.Date

Answer 3

You need a calendar table for this. You start with the calendar, and LEFT JOIN everything else, using BETWEEN logic.
You can use a real table. Or you can generate it on the fly, like this:
WITH
L0 AS ( SELECT c = 1
FROM (VALUES(1),(1),(1),(1),(1),(1),(1),(1),
(1),(1),(1),(1),(1),(1),(1),(1)) AS D(c) ),
L1 AS ( SELECT c = 1 FROM L0 A, L0 B, L0 C, L0 D ),
Nums AS ( SELECT rownum = ROW_NUMBER() OVER(ORDER BY (SELECT 1))
FROM L1 ),
Dates AS (
SELECT TOP (DATEDIFF(day, '20141231', GETDATE()))
Date = DATEADD(day, rownum, '20141231')
FROM Nums
)
SELECT
d.Date,
NumEmployed = COUNT(*)
FROM Dates d
JOIN YourTable t ON d.Date BETWEEN t.Hired AND t.Retired
GROUP BY
d.Date;
If your dates have a time component then you need to use >= AND < logic

Answer 4

Try limiting the scope of your date table. In this example I have a table of dates named TallyStickDT.
SELECT dt, COUNT(name)
FROM (
SELECT dt
FROM tallystickdt
WHERE dt >= (SELECT MIN(hired) FROM #employees)
AND dt <= GETDATE()
) A
LEFT OUTER JOIN #employees E ON A.dt >= E.Hired AND A.dt <= e.retired
GROUP BY dt
ORDER BY dt