Selecting unique pairs of equal and opposite rows

The question:

I am trying to select unique record “pairs”, where a pair is 2 rows that have equal and opposite numeric values (eg 10 and -10). Once a record has been paired, it can’t be used in another set of pairs. Consider this table:

    id text,
    scalar numeric

INSERT INTO vals (id, scalar)
    ('A', 10),
    ('B', -10),
    ('C', 10),
    ('D', -10),
    ('E', 10);

I have tried multiple variations of DISTINCT, DISTINCT ON (), and UNIQUE, but the closest I’ve come so far is the following:

WITH all_matching AS (
    SELECT id1, id2
    FROM vals v
    JOIN vals v2 ON
        v.scalar = (v2.scalar * -1)
    WHERE v.scalar > 0
), unique_left_id AS (
    FROM all_matching

SELECT * FROM unique_left_id;

Which outputs:

id1 | id2
 A  | B
 C  | B
 E  | B

The obvious problem is that the id B is being used to pair off against all of the other transactions, when I only want to use it once. If I select distinct on id2 after the final transaction, I would be left with a single pair instead of 2 pairs.

The Solutions:

Below are the methods you can try. The first solution is probably the best. Try others if the first one doesn’t work. Senior developers aren’t just copying/pasting – they read the methods carefully & apply them wisely to each case.

Method 1

cte AS (
    SELECT id, 
           ROW_NUMBER() OVER (PARTITION BY scalar ORDER BY id) rn
    FROM vals
FROM cte t1
JOIN cte t2 ON t1.scalar = -t2.scalar
           AND t1.scalar > t2.scalar
           AND t1.rn = t2.rn

PS. This code does not process scalar IN (0, NULL).

All methods was sourced from or, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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