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Query syntax

Posted on 2014-10-14
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Last Modified: 2014-10-28
Hello,

Can someone explain the different between the two lines of code marked below in terms of the way the grouping is:


with cte as (
 select 
  -1 as kli_kliniknr,
  max(m.MeasurementDate) as MeasurementDate, 
  SUM(NumberOfMeasurements) as NumberOfMeasurements,
  SUM(NumberOfMen) as NumberOfMen,
  SUM(NumberOfWomen) as NumberOfWomen,

  -- What is the difference between these two rows?
  avg( Q1 * NumberOfMeasurements / NumberOfMeasurements) as Q1,
  sum( Q1 * NumberOfMeasurements) / sum(NumberOfMeasurements) as Q1x,

from [BPSD].[dbo].[MeasurementStatisticsPerUnit] m
group by m.MeasurementDate )
 
select * from cte ;

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Question by:soozh
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LVL 22

Assisted Solution

by:plusone3055
plusone3055 earned 668 total points
ID: 40380750
-- What is the difference between these two rows?
  avg( Q1 * NumberOfMeasurements / NumberOfMeasurements) as Q1,
  sum( Q1 * NumberOfMeasurements) / sum(NumberOfMeasurements) as Q1x,

Row 1 is AVERAGING
Row2 is SUMING

Row1  AVERAGE (Q1 * number of measurements / number of mesaurements
Row2  (its adding all together)  SUM(q1 * numberof measurements / SUM(numberof measurements)
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LVL 48

Assisted Solution

by:Dale Fye
Dale Fye earned 664 total points
ID: 40380788
The AVG line is simply taking the average of the values of Q1.  The NumberOfMeasurements/Number of Measurements aspect of that element cancel each other out.

The Sum line looks like it is actually taking what most analysts would call the "weighted average".  This is determined by taking the sum of a value times the number of occurances of that value, and then dividing by the total number of measurements (the Sum(NumberOfMeasurements) figure).  So, if your table or query contains data like:

Q1     NumOfMeas
5                2
6                3
7               4

The first line would compute to the average of three elements)
AVG(5*2/2 , 6*3/3, 7*4/4) = 6

But the second method would compute as:
SUM(5*2, 6*3, 7*4)/Sum(2, 3, 4) = (10+18+28)/9 = 56/9 = 6.2222.
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LVL 70

Accepted Solution

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Scott Pletcher earned 668 total points
ID: 40380924
The first one is a simple average of Q1 ["* NumberOfMeasurements / NumberOfMeasurements" will always cancel each other out]

The second one, as Dale noted, is a different type of average, and could yield results quite different from the simple average.
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