Question

Question

• How do I write a T-SQL Stored Procedure that lets me select percentages of rows between X% and Y%?
• So basically I would want to select the rows between 30 PERCENT and 40 PERCENT…..

I know that you can do the following, but obviously that doesn’t let met specify a set of rows between 2 percentages.

SELECT TOP 50 PERCENT * FROM tblAssets 

Help greatly appreciated.

Answer 1

Updated Answer

declare @NumRecords int
SELECT @NumRecords = COUNT(*) FROM tblAssets;

With Vals As
(
SELECT tblAssets.AssetId ...
, ROW_NUMBER() OVER ( order by tblAssets.AssetId) as RN
  FROM tblAssets
)

SELECT  tblAssets.AssetId ...
FROM vals 
Where RN between 0.3*@NumRecords and 0.4*@NumRecords

I’ve updated my answer as there were 2 problems with my original answer below

1. Performance – It was beaten by the nested TOP solution
2. Accuracy – There is an unexpected aspect of NTILE that I was not aware of

If the number of rows in a partition
is not divisible by
integer_expression, this will cause
groups of two sizes that differ by one
member. Larger groups come before
smaller groups in the order specified
by the OVER clause. For example if the
total number of rows is 53 and the
number of groups is five, the first
three groups will have 11 rows and the
two remaining groups will have 10 rows
each.

I got the following values comparing with the nested TOP solution.

SET STATISTICS IO ON
SET STATISTICS TIME ON;

DECLARE @NumRecords int
SELECT @NumRecords = COUNT(*) FROM [master].[dbo].[spt_values];

WITH Vals As
(
SELECT  [number]
, ROW_NUMBER() OVER ( order by [number]) as RN
  FROM [master].[dbo].[spt_values]
)

SELECT [number] FROM vals Where RN
 BETWEEN 0.30*@NumRecords AND 0.40*@NumRecords

Gives

Table ‘spt_values’. Scan count 1,
logical reads 8, physical reads 0,
read-ahead reads 0, lob logical reads
0, lob physical reads 0, lob
read-ahead reads 0.

Table
‘spt_values’. Scan count 1, logical
reads 5, physical reads 0, read-ahead
reads 0, lob logical reads 0, lob
physical reads 0, lob read-ahead reads
0.

SELECT TOP 25 PERCENT [number] FROM
(
SELECT TOP 40 PERCENT  [number]
FROM  [master].[dbo].[spt_values]
ORDER BY [number]  ASC
) TOP40
ORDER BY [number] DESC

Gives

Table ‘Worktable’. Scan count 1,
logical reads 4726, physical reads 0,
read-ahead reads 0, lob logical reads
0, lob physical reads 0, lob
read-ahead reads 0.

Table
‘spt_values’. Scan count 1, logical
reads 8, physical reads 0, read-ahead
reads 0, lob logical reads 0, lob
physical reads 0, lob read-ahead reads
0.

Original Answer

With Vals As
(
SELECT tblAssets.AssetId ...
, NTILE (100)  OVER ( order by tblAssets.AssetId) as Pct
  FROM tblAssets 
)

SELECT * FROM vals Where Pct between 30 and 40