Question

I have a set of ids in a table (column id), lets say from 1-10, I want to couple.

I have an existing set of completely randomly generated couples in a table (columns id1, id2).

I want to find the set of remaining couples on a single SQL statement.

It’s very easy for permutations, my problem is with combinations without repetition. That means when considering (id1,id2) == (id2,id1) or in other words I don’t know how to do to find a set which doesn’t include the existing couples with the order changed.

Any ideas?

E.g. If i have table myids

|id|
|1 |
|2 |
|3 |
|4 |
|5 |
|6 |
|7 |
|8 |
|9 |
|10|

and table mycouples

|id1|id2|
|1  |2  |
|9  |1  |
|7  |8  |

I should get 10*9 – 6 = 84 different couples with my statement, that is all posible permutations of 10 elements not including (id1=1,id2=2),(id1=2,id2=1),(id1=9,id2=1),(id1=1,id2=9),(id1=7,id2=8),(id1=8,id2=7).

Answer 1

SELECT
    m1.id AS id1
  , m2.id AS id2
FROM myids AS m1
  JOIN myids AS m2
    ON m1.id <> m2.id
WHERE NOT EXISTS
      ( SELECT *
        FROM mycouples AS c
        WHERE (c.id1, c.id2) = (m1.id, m2.id)
      )
  AND NOT EXISTS
      ( SELECT *
        FROM mycouples AS c
        WHERE (c.id2, c.id1) = (m1.id, m2.id)
      )