Do I just not understand count()?

Er, uh, well, because I confused myself with my trivial example. :)

What I should have said was: it's giving me 3 (not 2), and I'm expecting 2
(not 1).

On Mon, 8 Apr 2002, Hunter Hillegas wrote:

I don't think I've seen that particular syntax used before (I would say
select count(a) from t where a=1;), but since the query appears to work, I
won't argue.

Why do you think it should give you a result of 1? There are two rows
containing a value of 1 for a, hence it returns 2.

Greg

----- Original Message -----
From: "Ben" <bench@silentmedia.com>
To: <pgsql-general@postgresql.org>
Sent: Monday, April 08, 2002 2:25 PM
Subject: [GENERAL] Do I just not understand count()?

That's what I'd normally do to, but in this case I want to run a query
more like

select count(a=1), count(a=2) from t

and I don't want to do multiple selects, because I'm selecting other stuff
too, which takes time, and I figure as long as postgres is looking at
those rows, it might as well tally up the counts of a=1 and a=2.

On Mon, 8 Apr 2002, Gregory Wood wrote:

I'm fairly sure you can't return a conditional count... count likes to
merely return the number of rows that match the query, which in this case is
all of them. I'm not sure exactly what it does with "a=1" though... I'm
guessing that it returns a boolean value, which means that it counts as a
row (only NULL values are not counted in count()).

If you're selecting other stuff, you won't be able to return more than one
row anyway... you'll have to either use a subquery or do a GROUP BY.

Greg

----- Original Message -----
From: "Ben" <bench@silentmedia.com>
To: "Gregory Wood" <gregw@com-stock.com>
Cc: "PostgreSQL-General" <pgsql-general@postgresql.org>
Sent: Monday, April 08, 2002 2:45 PM
Subject: Re: [GENERAL] Do I just not understand count()?

That's what I'd normally do to, but in this case I want to run a query
more like

select count(a=1), count(a=2) from t

and I don't want to do multiple selects, because I'm selecting other stuff
too, which takes time, and I figure as long as postgres is looking at
those rows, it might as well tally up the counts of a=1 and a=2.

On Mon, 8 Apr 2002, Gregory Wood wrote:

I don't think I've seen that particular syntax used before (I would say
select count(a) from t where a=1;), but since the query appears to work,

I

won't argue.

Why do you think it should give you a result of 1? There are two rows
containing a value of 1 for a, hence it returns 2.

Greg

----- Original Message -----
From: "Ben" <bench@silentmedia.com>
To: <pgsql-general@postgresql.org>
Sent: Monday, April 08, 2002 2:25 PM
Subject: [GENERAL] Do I just not understand count()?

If I have the table t defined as:

a
---
1
1
2

and I say:

select count(a=1) from t;

should it give me 1 or 2 as a result? I'm getting 2, and I'd think I
should get 1....

---------------------------(end of

broadcast)---------------------------

Try

select sum(case when a=1 then 1 else 0 end), sum(case when a=2 the 1 else 0
end) from t;

to get the number of occurrences of each value.

Jim Ballard

----- Original Message -----
From: "Clinton Adams" <clinton@vote-smart.org>
To: "Ben" <bench@silentmedia.com>
Cc: <pgsql-general@postgresql.org>
Sent: Monday, April 08, 2002 12:49 PM
Subject: Re: [GENERAL] Do I just not understand count()

If I have the table t defined as:

a
---
1
1
2

and I say:

select count(a=1) from t;

should it give me 1 or 2 as a result? I'm getting 2, and I'd think I
should get 1....

It will give you a result of 3, as a=1 does not toss out rows for which a

<>

Perhaps it would clear up things for those more expert on SQL [and it's
variations across DBs] than I if you could say on what system you would
normally do this.

That's what I'd normally do to, but in this case I want to run a query
more like

select count(a=1), count(a=2) from t

If I have the table t defined as:

a
---
1
1
2

and I say:

select count(a=1) from t;

should it give me 1 or 2 as a result? I'm getting 2, and I'd think I
should get 1....

--
Nigel J. Andrews
Director

---
Logictree Systems Limited
Computer Consultants

"Gregory Wood" <gregw@com-stock.com> writes:

I'm not sure exactly what it does with "a=1" though... I'm
guessing that it returns a boolean value, which means that it counts as a
row (only NULL values are not counted in count()).

Yup, Greg gets a gold star: COUNT *only* cares whether its input is NULL
or not, not what specific value it might have. This is per SQL spec.

The nearby suggestions involving SUM() look like they would work to
accumulate counts of different conditions in a single pass.

regards, tom lane

This is the first time I've tried. I noticed that I could add a DISTINCT
clause to count(), and wondered why I couldn't add an equality and make it
work as well.

On Mon, 8 Apr 2002, Nigel J. Andrews wrote: