Question

For example suppose I have the following join

join on p.match_id = o.match_id

Could I specify instead that if p.match_id is even then

join on p.match_id = o.match_id - 1

But if it is odd then

join on p.match_id = o.match_id + 1

I have sequential match_id such that consecutive values represent a single match. Therefore to work out an opponent for a given match_id I need the above

Answer 1

Edited so it doesn’t use CASE. The subquery returns a table with one field: matchid (equals match_id+1 when match_id is even and match_id-1 when match_id is odd).

SELECT matchid
  FROM ( SELECT match_id + 1 - 2*(match_id % 2)
                AS matchid
           FROM p
       ) AS p2
  JOIN o
    ON p2.matchid = o.match_id
;

Since you mention that you are using one table and 2 sequentials ids for one pair of opponents, you can also use the following as a subquery or a view:

SELECT match_id
     , ((match_id + 1) DIV 2) AS pair
  FROM p
;

It will give you a table that you can then collerate to itself:

| match_id | pair |
|     1    |   1  |
|     2    |   1  |
|     3    |   2  |
|     4    |   2  |
|     5    |   3  |
|     6    |   3  |