SQL Aggregate AVG function and GROUP BY Clause

I have a query that goes like this:

SELECT Avg([Boiler steam production]![CurrentSteamFlow]) AS [Average_lbs/hour]
FROM [Boiler steam production];

I assume that this is going to add all the fields in the CurrentSteamflow
column and average them....correct?

My next query goes like this:

SELECT Avg([Boiler steam production]![CurrentSteamFlow]) AS [Average_lbs/hour]
FROM [Boiler steam production]
GROUP BY [Boiler steam production].ReadingDate;

I expect this last query to average the CurrentSteamFlow column fields by
date ...... is this a correct assumption?
Right now I have 3 days worth of readings and this last query does group
them by ReadingDate, but if I add the 3 results up and divide by 3 it does
not equal the result of the first query...
Iam missing something here...any help would be greatly appreciated.
Thanks

Note that it seems from the alias (Average_lbs/hour) that these are hourly
readings. The first query is going to divide the total by 72 (3 days * 24
hours). The second is going to give you an average for each of the days. You
are then comparing the average of the averages to the first query. They are
not the same thing.

Are the numbers drastically different where you think this may be an SQL
problem? Or is this a difference that can be accounted for by rounding
errors? Don't forget- floating point math is not exact.

--

Chaim


"ktm400" wrote in message
...
I have a query that goes like this:

SELECT Avg([Boiler steam production]![CurrentSteamFlow]) AS
[Average_lbs/hour]
FROM [Boiler steam production];

I assume that this is going to add all the fields in the CurrentSteamflow
column and average them....correct?

My next query goes like this:

SELECT Avg([Boiler steam production]![CurrentSteamFlow]) AS
[Average_lbs/hour]
FROM [Boiler steam production]
GROUP BY [Boiler steam production].ReadingDate;

I expect this last query to average the CurrentSteamFlow column fields by
date ...... is this a correct assumption?
Right now I have 3 days worth of readings and this last query does group
them by ReadingDate, but if I add the 3 results up and divide by 3 it does
not equal the result of the first query...
Iam missing something here...any help would be greatly appreciated.
Thanks

Thanks for the reply Chaim.
The readings are taken every 15 minutes ....
The numbers are not drastically different and after going back and doing an
ave of each individual day, I realise what you are saying is true.
Thanks!

"Chaim" wrote:

Note that it seems from the alias (Average_lbs/hour) that these are hourly
readings. The first query is going to divide the total by 72 (3 days * 24
hours). The second is going to give you an average for each of the days. You
are then comparing the average of the averages to the first query. They are
not the same thing.

Are the numbers drastically different where you think this may be an SQL
problem? Or is this a difference that can be accounted for by rounding
errors? Don't forget- floating point math is not exact.

--

Chaim


"ktm400" wrote in message
...
I have a query that goes like this:

SELECT Avg([Boiler steam production]![CurrentSteamFlow]) AS
[Average_lbs/hour]
FROM [Boiler steam production];

I assume that this is going to add all the fields in the CurrentSteamflow
column and average them....correct?

My next query goes like this:

SELECT Avg([Boiler steam production]![CurrentSteamFlow]) AS
[Average_lbs/hour]
FROM [Boiler steam production]
GROUP BY [Boiler steam production].ReadingDate;

I expect this last query to average the CurrentSteamFlow column fields by
date ...... is this a correct assumption?
Right now I have 3 days worth of readings and this last query does group
them by ReadingDate, but if I add the 3 results up and divide by 3 it does
not equal the result of the first query...
Iam missing something here...any help would be greatly appreciated.
Thanks

And don't cross post! People will see questions in one place. If you cross
post:
1. You don't know where the answers will be posted. You have to look in
different places.
2. People will answer in one newsgroup only to find that the question was
already answered elsewhere. You have wasted their time, since you have the
answer and probably won't come back to read anything else posted to the
thread.

--

Chaim


"ktm400" wrote in message
...
I have a query that goes like this:

SELECT Avg([Boiler steam production]![CurrentSteamFlow]) AS
[Average_lbs/hour]
FROM [Boiler steam production];

I assume that this is going to add all the fields in the CurrentSteamflow
column and average them....correct?

My next query goes like this:

SELECT Avg([Boiler steam production]![CurrentSteamFlow]) AS
[Average_lbs/hour]
FROM [Boiler steam production]
GROUP BY [Boiler steam production].ReadingDate;

I expect this last query to average the CurrentSteamFlow column fields by
date ...... is this a correct assumption?
Right now I have 3 days worth of readings and this last query does group
them by ReadingDate, but if I add the 3 results up and divide by 3 it does
not equal the result of the first query...
Iam missing something here...any help would be greatly appreciated.
Thanks

sorry - new user, I thought I had posted the original post in the wrong forum
thus this cross post

"Chaim" wrote:

And don't cross post! People will see questions in one place. If you cross
post:
1. You don't know where the answers will be posted. You have to look in
different places.
2. People will answer in one newsgroup only to find that the question was
already answered elsewhere. You have wasted their time, since you have the
answer and probably won't come back to read anything else posted to the
thread.

--

Chaim


"ktm400" wrote in message
...
I have a query that goes like this:

SELECT Avg([Boiler steam production]![CurrentSteamFlow]) AS
[Average_lbs/hour]
FROM [Boiler steam production];

I assume that this is going to add all the fields in the CurrentSteamflow
column and average them....correct?

My next query goes like this:

SELECT Avg([Boiler steam production]![CurrentSteamFlow]) AS
[Average_lbs/hour]
FROM [Boiler steam production]
GROUP BY [Boiler steam production].ReadingDate;

I expect this last query to average the CurrentSteamFlow column fields by
date ...... is this a correct assumption?
Right now I have 3 days worth of readings and this last query does group
them by ReadingDate, but if I add the 3 results up and divide by 3 it does
not equal the result of the first query...
Iam missing something here...any help would be greatly appreciated.
Thanks