Re: Converting Rows to Columns

2009/5/5 Joshua Berry <yoberi@gmail.com>:
> Greetings all,
> I know that this is an elementary question, so I'm just asking for a pointer
> in the right direction.
> I have a query like this that tries to link Retail shops (from the table
> aliased as 'a') to sales reps (from the table aliased as 'e') that service
> the given Retail shop:
> select a.id_pdv, e.apellido ||', ' || e.nombre as seller_name from tbl_pdvs
> a join tbl_circuitos b using (id_circuito) join tbl_frecuencias c on
> b.id_circuito = c.id_circuito join tbl_rutas d on c.id_ruta = d.id_ruta join
> tbl_vendedores e on d.vendedor = e.id_vendedor order by a.id_pdv limit 4;
>
>  id_pdv  |   seller_name
> ---------+-------------------
>  1000001 | COLON, CRISTOBOL
>  1000001 | LOPEZ, CARLOS
>  1000002 | COLON, CRISTOBOL
>  1000002 | LUGO, FERNANDO
>
> As you can see, there are two salesmen that service this shop.
> What I need is to be able to create a report that has each unique id_pdv as
> a row of the result set, with three columns available to show up to three
> salemen that are assigned to the route.
> So this would give something like this:
>  id_pdv  |  seller_name1    | seller_name2   | seller_name3
> ---------+------------------+----------------+----------------
>  1000001 | COLON, CRISTOBOL | LOPEZ, CARLOS  | (\N or '')
>  1000002 | COLON, CRISTOBOL | LUGO, FERNANDO | (\N or '')
>
> Note that the order of the sellers does not matter.
> Any tips? I've googled 'sql convert rows to columns' and got some results
> that appear to be mssql specific. On Stackoverflow there is an example of
> how to convert columns to rows using UNION, but not visa versa.

Quick and dirty solution off the top of my head for the problem described:

CREATE TABLE salesperson (
  id SERIAL PRIMARY KEY,
  name TEXT
);

CREATE TABLE salesperson_store (
  salesperson_id INT NOT NULL,
  store_id INT NOT NULL,
  PRIMARY KEY (salesperson_id, store_id)
);

INSERT INTO salesperson VALUES
  (1, 'COLON, CRISTOBOL'),
  (2, 'OPEZ, CARLOS'),
  (3, 'LUGO, FERNANDO');

INSERT INTO salesperson_store VALUES
  (1, 1000001),
  (1, 1000002),
  (2, 1000001),
  (3, 1000002);

SELECT store_id,
       (SELECT sp.name FROM salesperson sp
          INNER JOIN salesperson_store sps
           ON sp.id=sps.salesperson_id
           AND sps.store_id=store.store_id
        ORDER BY sp.name LIMIT 1 OFFSET 0)
        AS seller_name1,
         (SELECT sp.name FROM salesperson sp
          INNER JOIN salesperson_store sps
           ON sp.id=sps.salesperson_id
           AND sps.store_id=store.store_id
          ORDER BY sp.name LIMIT 1 OFFSET 1)
        AS seller_name2,
       (SELECT sp.name FROM salesperson sp
          INNER JOIN salesperson_store sps
           ON sp.id=sps.salesperson_id
           AND sps.store_id=store.store_id
           ORDER BY sp.name LIMIT 1 OFFSET 2)
        AS seller_name3
  FROM (SELECT DISTINCT(store_id) FROM salesperson_store) store
ORDER BY store_id;

 store_id |   seller_name1   |  seller_name2  | seller_name3
----------+------------------+----------------+--------------
  1000001 | COLON, CRISTOBOL | OPEZ, CARLOS   |
  1000002 | COLON, CRISTOBOL | LUGO, FERNANDO |
(2 rows)

(No doubt there are probably more elegant ways of doing this)

HTH

Ian Barwick