Simulating MySQL's ORDER BY FIELD() in Postgresql

Question

Just trying out PostgreSQL for the first time, coming from MySQL. In our Rails application we have a couple of locations with SQL like so:

SELECT * FROM `currency_codes` ORDER BY FIELD(code, 'GBP', 'EUR', 'BBD', 'AUD', 'CAD', 'USD') DESC, name ASC

It didn't take long to discover that this is not supported/allowed in PostgreSQL.

Does anyone know how to simulate this behaviour in PostgreSQL or do we have to pull sorting out into the code?

Answer 1

Ah, gahooa was so close:

SELECT * FROM currency_codes
  ORDER BY
  CASE
    WHEN code='USD' THEN 1
    WHEN code='CAD' THEN 2
    WHEN code='AUD' THEN 3
    WHEN code='BBD' THEN 4
    WHEN code='EUR' THEN 5
    WHEN code='GBP' THEN 6
    ELSE 7
  END,name;

Answer 2

sort in mysql:

> ids = [11,31,29]
=> [11, 31, 29]
> User.where(id: ids).order("field(id, #{ids.join(',')})")

in postgres:

def self.order_by_ids(ids)
  order_by = ["CASE"]
  ids.each_with_index do |id, index|
    order_by << "WHEN id='#{id}' THEN #{index}"
  end
  order_by << "END"
  order(order_by.join(" "))
end

User.where(id: [3,2,1]).order_by_ids([3,2,1]).map(&:id) 
#=> [3,2,1]

Answer 3

Update, fleshing out terrific suggestion by @Tometzky.

This ought to give you a MySQL FIELD()-alike function under pg 8.4:

-- SELECT FIELD(varnames, 'foo', 'bar', 'baz')
CREATE FUNCTION field(anyelement, VARIADIC anyarray) RETURNS numeric AS $$
  SELECT
    COALESCE(
     ( SELECT i FROM generate_subscripts($2, 1) gs(i)
       WHERE $2[i] = $1 ),
     0);
$$ LANGUAGE SQL STABLE

Mea culpa, but I cannot verify the above on 8.4 right now; however, I can work backwards to a "morally" equivalent version that works on the 8.1 instance in front of me:

-- SELECT FIELD(varname, ARRAY['foo', 'bar', 'baz'])
CREATE OR REPLACE FUNCTION field(anyelement, anyarray) RETURNS numeric AS $$
  SELECT
    COALESCE((SELECT i
              FROM generate_series(1, array_upper($2, 1)) gs(i)
              WHERE $2[i] = $1),
             0);
$$ LANGUAGE SQL STABLE

More awkwardly, you still can portably use a (possibly derived) table of currency code rankings, like so:

pg=> select cc.* from currency_codes cc
     left join
       (select 'GBP' as code, 0 as rank union all
        select 'EUR', 1 union all
        select 'BBD', 2 union all
        select 'AUD', 3 union all
        select 'CAD', 4 union all
        select 'USD', 5) cc_weights
     on cc.code = cc_weights.code
     order by rank desc, name asc;
 code |           name
------+---------------------------
 USD  | USA bits
 CAD  | Canadian maple tokens
 AUD  | Australian diwallarangoos
 BBD  | Barbadian tridents
 EUR  | Euro chits
 GBP  | British haypennies
(6 rows)