Обсуждение: create aggregate function 'count_bool( column_name, boolean )'

James,

I know Postgresql doesn't have 2 arg aggregate functions.  what you could do is the following (untested)

select distict product_id, 
sum(case when purchased then 1 else 0 end) as purchased,
sum(case when was_selected then 1 else 0 end) as was_selected
from some_table 
group by product_id;

Jim

---------- Original Message -----------
From: "James Moliere" <jmoliere@ucsd.edu>
To: <pgsql-sql@postgresql.org>
Sent: Fri, 7 Jul 2006 06:53:45 -0700
Subject: [SQL] create aggregate function 'count_bool( column_name, boolean )'

> Hello,
> I'd like to create a function called count_bool( column_name, boolean ) in
> PostgreSQL.
> 
> this function is similar to the count( column_name ) function but will only
> count the trues or falses based on the other input parameter boolean. e.g.
> if you pass in a 'true', all the trues will be counted but not the falses --
> it's the same but opposite if the 'false' was passed in.
> 
> I'd like this aggregate function to be developed with the SQL language
> instead of 'C' (for maintenance reasons).  From the surface, it appears to
> be an incredibly simple job only to find that it's difficult.
> 
> In some respects, I can't believe this function doesn't exist in SQL
> 
> so now I can create the statement
> select distict product_id, count_bool(purchased, true),
> count_bool(was_selected, true) from some_table group by product_id;
> 
> instead of breaking the query into 3 seperate queries
> 
> select distict product_id from some_table;
> select count(purchased) from product_id where purchased = true;
> select count(was_selected) from some_table where was_selected = true;
> 
> Am I missing a detail with SQL based aggregate function development?
> 
> Any help would be appreciated.
> 
> Thanks!
------- End of Original Message -------

> so now I can create the statement
> select distict product_id, count_bool(purchased, true),
> count_bool(was_selected, true) from some_table group by product_id;
> 
> instead of breaking the query into 3 seperate queries
> 
> select distict product_id from some_table;
> select count(purchased) from product_id where purchased = true;
> select count(was_selected) from some_table where was_selected = true;
> Am I missing a detail with SQL based aggregate function development?
> Any help would be appreciated.

how about:

select    product_id,    (     select count(purchased)     from some_table as A2     where purchased=true and
A1.product_id=A2.product_id  ) as TP,   (     select count(selected)     from some_table as A3     where purchased=true
andA1.product_id=A3.product_id   ) as TS
 
from some_table as A1
group by product_id;

Regards,

Richard Broersma Jr.