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Re: Question About Aggregate Functions

From: "Brandon Aiken" <BAiken(at)winemantech(dot)com>
To: <pgsql-novice(at)postgresql(dot)org>
Subject: Re: Question About Aggregate Functions
Date: 2006-09-13 13:34:21
Message-ID: F8E84F0F56445B4CB39E019EF67DACBA2F1EC8@exchsrvr.winemantech.com (view raw or flat)
Thread:
Lists: pgsql-generalpgsql-novice
Ah, I did not know what was in your fields, so I did not assume they
were Boolean values.  It looked to me like you were trying to use IS
TRUE to substitute for the lack of a GROUP BY, so I didn't know what to
do.

 

Yes, count() will include all non-NULL values.  Sorry if I sounded
unclear there.  

 

If you do typecasting the value zero is false (and non-zero is true).
NULL in an expression always returns NULL, and many programs will
interpret that result as false.  So I'm not sure of what results you
might get with a Boolean test against a non-Boolean field, especially if
it's an integer field.

 

postgres=# select 0::boolean = FALSE;

 ?column?

----------

 t

(1 row)

 

You should just be able to take the previous query and add in your WHERE
clauses:

 

SELECT count(t1.fielda), count(t2.fielda), count(t2.fieldb),
AVG(t2.fieldc)

FROM t1 JOIN t2 ON t1.item_id = t2.item_id

WHERE t1.fielda = TRUE AND t2.fielda = TRUE AND t2.fieldb = TRUE

GROUP BY NULL;

 

Now, the INNER JOIN you're using is only selecting fields where both
t1.item_id and t2.item_id exist and the respective fields are TRUE.
That is, it's only going to run the count and average functions against
the results of this query:

SELECT *

FROM t1 JOIN t2 ON ON t1.item_id = t2.item_id

WHERE t1.fielda = TRUE AND t2.fielda = TRUE AND t2.fieldb = TRUE;

 

If that's what you want, that's great.  

 

However, you might want a count of each field where that field is TRUE.
In that case, I would use either temporary tables, compound queries and
derived tables, or multiple simple queries.

 

It's also possible that you might want a count of fields where
t1.item_id and t2.item_id exist, but where only each respective field is
TRUE.  That is, you want a count of t1.fielda where it is TRUE no matter
what t2.fielda and t2.fieldb are as long as t1.item_id matches
t2.item_id.  In that case you have to do even more joins, and that could
take a fair bit of time especially if you haven't indexed your item_id
fields.

 

You really have to look at your result sets.  Sometimes it is better to
run multiple simple queries instead of one big complex query to be sure
you're getting the data you want and the query executes in a reasonable
amount of time.

 

Also, consider that NULL values are generally considered bad to
purposefully enter.  Logically, It would be better to create one table
for each field and then create a record for each item_id as you need it
so you never have NULLs.  The problem with that is one of performance if
you end up doing large number of JOINs.  In that case, it might be
better to use integers instead of Boolean fields, since you have three
explicit states of TRUE, FALSE, and NOT YET DETERMINED.

 

--

Brandon Aiken

CS/IT Systems Engineer

________________________________

From: pgsql-novice-owner(at)postgresql(dot)org
[mailto:pgsql-novice-owner(at)postgresql(dot)org] On Behalf Of Don Parris
Sent: Tuesday, September 12, 2006 9:16 PM
To: pgsql-novice(at)postgresql(dot)org
Subject: Re: [NOVICE] Question About Aggregate Functions

 

On 9/12/06, Brandon Aiken <BAiken(at)winemantech(dot)com> wrote:

	First, aggregate functions always have to have a GROUP BY
clause.  If you want everything in a table or join, you use GROUP BY
NULL.  


Thanks.  I did not realize that. 

	 

	Next, IS TRUE statements will select anything that is not NULL,
0, or FALSE, so I'm not sure what you're trying to get because you're
getting nearly everything, and count() already ignores NULL values.


I didn't see that in the manual's coverage, but could have overlooked
it.  But count() will include the FALSE values along with the TRUE
values - ignoring only those that are NULL.  At least, I think that's
the case.  So, for each column I select, I need to be sure I am counting
only the TRUE values.  I do have NULL, FALSE and TRUE values in each
column, since I do not always know for sure whether an attribute is TRUE
or FALSE when I record the item.  That may be determined later, but not
in all cases. 

	 

	Next, count(x, y, z) isn't a valid function.  Count() only has
one parameter, so you'll have to call it several times.


I knew my syntax was wrong - but wasn't sure about calling multiple
functions since I hadn't seen any examples of that in my hunting for
info.  I was trying to make a little clearer what I wanted to do. 
 

	 

	Depending on what you were hoping count(x, y, z) was returning,
you do this: 

	 

	SELECT count(t1.fielda), count(t2.fielda), count(t2.fieldb),
AVG(t2.fieldc)

	FROM t1 JOIN t2 ON ON t1.item_id = t2.item_id

	GROUP BY NULL;


This one looks more like what I am attempting to do.  However, I do need
to be sure my  count() functions are counting the values that are TRUE.
Is this a case where I should run a query to select the records where
the values for the desired columns are true, insert that result into a
temp table, and then perform the count() function as above on just those
records?  Sure seems like that would be the simple route, now that I
think about it. 

 


 

<SNIP>

Greetings,

I'm a DB novice as well as a pgsql novice.  I can manage to run a few
basic queries, but that's all I've really done so far.  How do I create
a query that (1) evaluates each boolean field for TRUE/FALSE and (2)
counts the number of rows where each field is TRUE?  Also, one field is
an integer, so I want the average from that field, rather than the
count.  Something along the lines of: 

SELECT COUNT (t1.fielda, t2.fielda, t2.fieldb) AVG(t2.fieldc) FROM t1,
t2
WHERE t1.item_id=t2.item_id AND t1.fielda IS TRUE  AND t2.fielda IS TRUE
AND t2.fieldb IS TRUE AND t2.fieldc IS NOT NULL

The result is intended to be something of a summary report.  t1 contains
basic info about each item, with one field for whether the item is
active or not.  The other table contains additional info about whether
each item has particular attributes.  I want the query to tell me the
average number of years active items have been active, and the number of
items where each attribute is true.  I then want to turn the raw
attribute counts into percentages, so I can say "for n% of the items
these attributes are true". 

Pointers to good examples/tutorials are welcome.  Most I have seen are
rather simplistic, and what I am reading in the manual isn't coming
together very well.

Thanks,
Don

 

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