# Selecting unique pairs of equal and opposite rows

## All we need is an easy explanation of the problem, so here it is.

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:

``````CREATE TABLE vals (
id text,
scalar numeric
);

INSERT INTO vals (id, scalar)
VALUES
('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
v.id id1,
v2.id id2
FROM vals v
JOIN vals v2 ON
v.scalar = (v2.scalar * -1)
WHERE v.scalar > 0
), unique_left_id AS (
SELECT DISTINCT ON (id1) *
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.

## How to solve :

I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.

### Method 1

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

https://dbfiddle.uk/?rdbms=postgres_12&fiddle=2561e29b69b587e8c827169b361bde07

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

Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂

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