How to order some rows one way, and the rest another way?

Question

I am trying to sort rows from a table in PostgreSQL 10.12:

CREATE TABLE example (
 id  serial PRIMARY KEY,
 val text NOT NULL,
 x   integer NOT NULL,
 y   integer NOT NULL
);
    
INSERT INTO example (val,x,y)
VALUES
  ('First Value (This should be first order)',7,2)
, ('Second Value (This should be second order)',6,3)
, ('Third Value (This should be third order)',5,4)
, ('Seventh Value (And This should be last order)',4,1)
, ('Sixth Value (This should be sixth order)',3,5)
, ('Fifth Value (This should be fifth order)',2,6)
, ('Fourth Value (This should be fourth order)',1,7)
;

First three result rows should be ordered by x desc, and the rest should be ordered by y desc.

I tried this query but it is ordering only by y:

SELECT * from (SELECT * from example order by x desc fetch first 3 rows only) foo
UNION
SELECT * from example order by y desc;

But it sorts by only y. And I need to get the result without UNION.

Here is an SqlFiddle.

Answer 1

This returns the top 3 rows according to ORDER BY x DESC and appends the rest sorted by y DESC:

WITH cte AS (TABLE example ORDER BY x DESC LIMIT 3)
TABLE cte
UNION ALL
(  -- parentheses required
SELECT e.*
FROM   example e
LEFT   JOIN cte USING (id)
WHERE  cte.id IS NULL
ORDER  BY e.y DESC
);

TABLE is just shorthand for SELECT * FROM.

UNION ALL makes sure that the two derived tables are just appended without trying to eliminate (non-existent) duplicates and thereby messing with the sort order.

Parentheses are required so that ORDER BY only applies enclosed SELECT, not to the complete set of rows.

See:

• Combine multiple SELECT statements

• Is order preserved after UNION in PostgreSQL?

Related:

• Combining 2 SELECT queries and printing the results in PostgreSQL

• Is there a faster way to get UNION ALL behavior in Postgres?

Without UNION

SELECT e.*
FROM   example  e
LEFT   JOIN (
   SELECT id, x FROM example ORDER BY x DESC LIMIT 3
   ) c USING (id)
ORDER  BY c.x DESC NULLS LAST, e.y DESC;

Same result.

The LEFT JOIN results in c.x being NULL except for the top 3 chosen rows. So the first ORDER BY items c.x DESC NULLS LAST only sorts the first 3 rows and leaves the rest unsorted. The second ORDER BY item e.y DESC sorts the rest as desired.

About NULLS LAST:

• Sort by column ASC, but NULL values first?

db<>fiddle here

If the table is not trivially small, you should have and index on (x) and another one on (y)

Answer 2

You could try a common table expression:

with    cte_example
            as
            (
                select row_number() over (order by x desc) as my_row_count
                , id, val, x, y
                from example
            )   
            select my_row_count, id, val, x, y
            from cte_example
            where my_row_count <= 3;