T-SQL Question - Counting and Average - sql

Question

I have a set of data that consists of a filenbr, open date and close date.
I need to produce a summary table similar to the below, i need to count how many files belong to each day period, but i need those greater than 20 grouped together. I know how to get the datediff, what i'm stumbling on is how to get the 20+ and the % column
1 day - 30 files - 30%
3 days - 25 files - 25%
10 days - 5 files - 5%
13 days - 20 files - 20%
>= 20 days - 20 files - 20%

Answer 1

suppose you have a table named dayFile with the following columns
Table DayFile
days - files
1 - 10
1 - 5
1 - 15
3 - 20
3 - 5
10 - 5
13 - 20
20 - 5
22 - 5
28 - 10
Then you could do the following
SELECT
SummaryTable.Day,
SUM(SummaryTable.Files) as SumFiles,
Sum(SummaryTable.Files) / SummaryTable.TotalFiles
FROM
(SELECT
CASE WHEN (days >= 20) THEN
20
ELSE DF.days END as Day
files,
(SELECT Count(*) FROM DayFile DFCount) as TotalFiles
FROM DayFile DF) SummaryTable
Group By SummaryTable.Day
EDITED:
SELECT
SummaryTable.Day,
SUM(SummaryTable.Files) as SumFiles,
Sum(SummaryTable.Files) / (SELECT Count(*) FROM DayFile DFCount)
FROM
(SELECT
CASE WHEN (days >= 20) THEN
20
ELSE DF.days END as Day
files
FROM DayFile DF) SummaryTable
Group By SummaryTable.Day

Answer 2

You are unclear as to how the ranges are determined (e.g. does "3 days mean < 3 days, <= 3 days, = 3 days, > 3 days or >= 3 days?). If you are using SQL Server 2005 and higher, you get your results like so:
With PeriodLength As
(
Select DateDiff(d, OpenDate, CloseDate) As DiffDays
From Table
)
, Ranges As
(
Select Case
When DiffDays < 3 Then 'Less than 3 Days'
When DiffDays >= 3 And DiffDays < 10 Then 'Less than 10 Days'
When DiffDays >= 10 And DiffDays < 13 Then 'Less than 13 Days'
When DiffDays >= 13 And DiffDays < 20 Then 'Less than 20 Days'
When DiffDays >= 20 Then 'Greater than 20 days'
End As Range
From PeriodLength
)
Select Range
, Count(*) As FileCount
, Count(*) * 100.000 / (Select Count(*) From Ranges) As Percentage
From Ranges
Group By Range