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,emp4app1 |     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.lang@attiksystem.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
>
>
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