Re: [HACKERS] need help on writing an aggregate function

[Bruno Wolff III]

On Tue, Nov 16, 2004 at 12:41:45 -0700,
  [EMAIL PROTECTED] wrote:
> hi,
> I was writing my own data type, and, I have to write aggregare function like
> min() and max() for my datatype. I googled for the user defined aggregate
> functions, but I could not find any examples for the sfunc and ffunc. 
> Can any of you provide me the source/structure in C or SQL for these two
> functions of min or max or avg etc? for a complex type as given in the 
> examples.
> Thanks,
> subhash. 

Here is an example I did to do concatenation:

drop view  people_with_email;
drop table people2email;
drop table email;
drop table people;
drop aggregate concatenate(text);
drop function join_with_comma(text,text);

create function join_with_comma(text,text)
  returns text
  immutable strict language 'sql'
  as 'select $1||'', ''||$2'
;

create aggregate concatenate (
  sfunc = join_with_comma,
  basetype = text,
  stype = text
);

create table email (
    email_id        integer primary key,
    email_address   text not null unique
);

copy email from stdin with delimiter '|';
1|[EMAIL PROTECTED]
2|[EMAIL PROTECTED]
3|[EMAIL PROTECTED]
4|[EMAIL PROTECTED]
\.

create table people (
    person_id       integer primary key,
    first_name      text not null,
    last_name       text not null
);

copy people from stdin with delimiter '|';
1|Scott|Goodwin
2|Fred|Flintstone
3|Barney|Rubble
\.

create table people2email (
    person_id       integer references people (person_id),
    email_id        integer references email (email_id)
);

copy people2email from stdin with delimiter '|';
1|1
2|2
3|3
3|4
\.

create view people_with_email as
   select
       a.first_name,
       a.last_name,
       c.email_address
   from
       people a,
       (select r.person_id, concatenate(b.email_address) as email_address
           from people2email r, email b
           where r.email_id = b.email_id
           group by r.person_id) as c
    where a.person_id = c.person_id
;

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