Query syntax

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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soozhCEOAsked:
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plusone3055Commented:
-- 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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Dale FyeCommented:
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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Scott PletcherSenior DBACommented:
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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Query Syntax

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