Simulating MySQL's ORDER BY FIELD() in Postgresql
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;
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]
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)