SQL ORDER BY the 'IN' value list

Question

I have a simple SQL query (in PostgreSQL 8.3) that grabs a bunch of comments. I've composed a bunch of ids before-hand and that gets fed into the WHERE IN clause like so...

SELECT * FROM "comments" WHERE ("comments"."id" IN (1,3,2,4))

This returns the comments in a natural order which in my case is the ids like 1,2,3,4.

What I'm wanting is to sort the returned rows in the order supplied by the IN clause (1,3,2,4).

How can I ORDER BY the IN clause value list?

Answer 1

I see no way to do this with straight SQL. I mean, at least not without a lot of blood sweat and tears.

Answer 2

On researching this some more I found this solution:

SELECT * FROM "comments" WHERE ("comments"."id" IN (1,3,2,4)) 
ORDER BY CASE "comments"."id"
WHEN 1 THEN 1
WHEN 3 THEN 2
WHEN 2 THEN 3
WHEN 4 THEN 4
END

However this seems rather verbose and might have performance issues with large datasets. Can anyone comment on these issues?

Answer 3

To do this, I think you should probably have an additional "ORDER" table which defines the mapping of IDs to order (effectively doing what your response to your own question said), which you can then use as an additional column on your select which you can then sort on.

In that way, you explicitly describe the ordering you desire in the database, where it should be.

Answer 4

I agree with all other posters that say "don't do that" or "SQL isn't good at that". If you want to sort by some facet of comments then add another integer column to one of your tables to hold your sort criteria and sort by that value. eg "ORDER BY comments.sort DESC " If you want to sort these in a different order every time then... SQL won't be for you in this case.

Answer 5

SELECT * FROM "comments" JOIN (
  SELECT 1 as "id",1 as "order" UNION ALL 
  SELECT 3,2 UNION ALL SELECT 2,3 UNION ALL SELECT 4,4
) j ON "comments"."id" = j."id" ORDER BY j.ORDER

or if you prefer evil over good:

SELECT * FROM "comments" WHERE ("comments"."id" IN (1,3,2,4))
ORDER BY POSITION(','+"comments"."id"+',' IN ',1,3,2,4,')

Answer 6

create sequence serial start 1;

select * from comments c
join (select unnest(ARRAY[1,3,2,4]) as id, nextval('serial') as id_sorter) x
on x.id = c.id
order by x.id_sorter;

drop sequence serial;

[EDIT]

unnest is not yet built-in in 8.3, but you can create one yourself(the beauty of any*):

create function unnest(anyarray) returns setof anyelement
language sql as
$$
    select $1[i] from generate_series(array_lower($1,1),array_upper($1,1)) i;
$$;

that function can work in any type:

select unnest(array['John','Paul','George','Ringo']) as beatle
select unnest(array[1,3,2,4]) as id

Answer 7

And here's another solution that works and uses a constant table (http://www.postgresql.org/docs/8.3/interactive/sql-values.html):

SELECT * FROM comments AS c,
(VALUES (1,1),(3,2),(2,3),(4,4) ) AS t (ord_id,ord)
WHERE (c.id IN (1,3,2,4)) AND (c.id = t.ord_id)
ORDER BY ord

But again I'm not sure that this is performant.

I've got a bunch of answers now. Can I get some voting and comments so I know which is the winner!

Thanks All :-)

Answer 8

sans SEQUENCE, works only on 8.4:

select * from comments c
join 
(
    select id, row_number() over() as id_sorter  
    from (select unnest(ARRAY[1,3,2,4]) as id) as y
) x on x.id = c.id
order by x.id_sorter

Answer 9

You can do it quite easily with (introduced in PostgreSQL 8.2) VALUES (), ().

Syntax will be like this:

select c.*
from comments c
join (
  values
    (1,1),
    (3,2),
    (2,3),
    (4,4)
) as x (id, ordering) on c.id = x.id
order by x.ordering

Answer 10

Slight improvement over the version that uses a sequence I think:

CREATE OR REPLACE FUNCTION in_sort(anyarray, out id anyelement, out ordinal int)
LANGUAGE SQL AS
$$
    SELECT $1[i], i FROM generate_series(array_lower($1,1),array_upper($1,1)) i;
$$;

SELECT 
    * 
FROM 
    comments c
    INNER JOIN (SELECT * FROM in_sort(ARRAY[1,3,2,4])) AS in_sort
        USING (id)
ORDER BY in_sort.ordinal;