Re: Concatenation through SQL

Hello

postgres=# select name, count, employes from (select appointment_id,
count(*), (SELECT array_to_string(ARRAY(SELECT name from employees
where appointment_id = e.appointment_id),',')) as employes from
employees e group by appointment_id)s join appointments a on a.id =
s.appointment_id;
name | count | employes
------+-------+----------------
app2 | 2 | emp1,emp4
app1 | 3 | emp1,emp2,emp3
(2 rows)

regards
Pavel Stehule

for longer table is better define own aggregate function.

On 21/12/2007, Philippe Lang <philippe(dot)lang(at)attiksystem(dot)ch> wrote:
> Hi,
>
> Here is small reduced test database:
>
> ------------------------------
> CREATE TABLE appointments
> (
> id integer,
> name varchar(32),
> CONSTRAINT appointments_pkey PRIMARY KEY (id)
> );
>
> CREATE TABLE employees
> (
> id integer,
> appointment_id integer,
> name varchar(32),
> CONSTRAINT employees_pkey PRIMARY KEY (id),
> CONSTRAINT appointments_employees_fkey FOREIGN KEY (appointment_id)
> REFERENCES appointments (id)
> );
>
> INSERT INTO appointments VALUES (1, 'app1');
> INSERT INTO employees VALUES (1, 1, 'emp1');
> INSERT INTO employees VALUES (2, 1, 'emp2');
> INSERT INTO employees VALUES (3, 1, 'emp3');
>
> INSERT INTO appointments VALUES (2, 'app2');
> INSERT INTO employees VALUES (4, 2, 'emp1');
> INSERT INTO employees VALUES (5, 2, 'emp4');
> ------------------------------
>
>
> I'm trying to write an SQL query that would return this:
>
> ---------------------------------------------------
> appointment count_employees employees
> ---------------------------------------------------
> app1 3 emp1, emp2, emp3
> app2 2 emp1, emp4,
> ---------------------------------------------------
>
> First part is easy to write:
>
> ------------------------------
> SELECT
> appointments.name AS appointment,
> (SELECT COUNT(*) FROM employees AS e where e.appointment_id =
> appointments.id) AS num_employees
> FROM appointments
> ------------------------------
>
> ... But concatenating employees name is harder, at least for me...
>
> I'm convinced this can be done with Set Returning Functions and a bit of
> plpgsql, but I was wondering if it would possible to write some kind of
> extension to Postgresql (operator?) that would allow this kind of
> syntax:
>
> ------------------------------
> SELECT
> appointments.name AS appointment,
> (SELECT COUNT(*) FROM employees AS e where e.appointment_id =
> appointments.id) AS num_employees
> (SELECT CONCAT(name, ', ') FROM employees AS e where e.appointment_id =
> appointments.id) AS employees
> FROM appointments
> ------------------------------
>
> ... where CONCAT suggest we want to concatenate the variable inside,
> with the separator ', ' inbetween.
>
> Thanks for your tips!
>
>
> Philippe Lang
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 7: You can help support the PostgreSQL project by donating at
>
> http://www.postgresql.org/about/donate
>