How to calculate average of variables in SQL Server - sql

Question

I am trying to calculate date ranges between rows and then get an average.
declare #date0 date = (
select top 1 my_date
from someTable
order by my_date desc)
declare #date1 date = (
select my_date
from someTable
order by my_date desc
offset 1 rows
fetch next 1 row only)
declare #date2 date = (
select my_date
from someTable
order by my_date desc
offset 2 rows
fetch next 1 row only)
declare #date3 date = (
select my_date
from someTable
order by my_date desc
offset 3 rows
fetch next 1 row only)
select
[Range 1] = dateDiff(day, #date1, #date0),
[Range 2] = dateDiff(day, #date2, #date1),
[Range 3] = dateDiff(day, #date3, #date2),
[Avg Range] = avg(
nullIf(#date0, 0),
nullIf(#date1, 0),
nullIf(#date2, 0),
nullIf(#date3, 0)
)
The range calculations work fine, but a bit clumsy.
However, I'm not sure how to handle the average. It looks like the function is supposed to run against a table and not an array, but I was having issues inserting the variables into a temp table column.
How can I get the average of these ranges (not including range = 0)?

Answer 1

Use UNION ALL to return a row for each case:
select avg(t.[Range]) [Avg Range]
from (
select dateDiff(day, #date1, #date0) [Range]
union all
select dateDiff(day, #date2, #date1)
union all
select dateDiff(day, #date3, #date2)
) t
where t.[Range] <> 0

Answer 2

AVG is an aggregate function, designed to be used with GROUP BY or windows. You can simply do the math in your query:
select
[Range 1] = dateDiff(day, #date1, #date0),
[Range 2] = dateDiff(day, #date2, #date1),
[Range 3] = dateDiff(day, #date3, #date2),
[Avg Range] = (
nullIf(#date0, 0) +
nullIf(#date1, 0) +
nullIf(#date2, 0) +
nullIf(#date3, 0)
) /
(
CASE WHEN #date0 IS NULL THEN 0 ELSE 1 END +
CASE WHEN #date1 IS NULL THEN 0 ELSE 1 END +
CASE WHEN #date2 IS NULL THEN 0 ELSE 1 END +
CASE WHEN #date3 IS NULL THEN 0 ELSE 1 END
)

Answer 3

No reason to use four different queries:
with dates as (
select
row_number() over (order by my_date desc) rn,
datediff(days, lag(my_date) over (order by my_date desc), my_date) diff
from T
)
select avg(nullif(diff, 0)) from dates where rn <= 3;
or
with dates as (
select
datediff(days, lag(my_date) over (order by my_date desc), my_date) diff
from T
order by my_date desc
fetch next three rows only
)
select avg(nullif(diff, 0)) from dates;
Using distinct would also let you easily get the top three dates and not have to mess around with nullif().
with dates as (
select distinct
datediff(days, lag(my_date) over (order by my_date desc), my_date) diff
from T
order by my_date desc
fetch next three rows only
)
select avg(diff) from dates;