Question

Consider the following table:

 id | x  |  y  
----+----+-----
  1 | 10 | 100
  1 | 20 | 120
  1 | 30 | 130
  2 | 10 | 130
  2 | 20 | 130
  2 | 30 | 130
  3 | 20 | 130
(7 rows)

I would like to have a single arbitrary tuple of every id, with the corresponding x and y values. The following results are correct:

 id | x  |  y  
----+----+-----
  1 | 10 | 100
  2 | 20 | 130
  3 | 20 | 130

Or:

 id | x  |  y  
----+----+-----
  1 | 30 | 130
  2 | 30 | 130
  3 | 20 | 130

Neither id, x nor y are unique in any context.

How do I go about this in SQL (under PostgreSQL)? There might be some millions of ids.

Update:

Thanks, @Lukáš Lalinský. Quoting the PostgreSQL documentation you’ve linked to:

DISTINCT ON ( expression [, …] ) keeps only the first row of each
set of rows where the given expressions evaluate to equal… Note that the “first row” of each set is
unpredictable unless ORDER BY is used to ensure that the desired row
appears first.

Answer 1

Using the DISTINCT ON clause should do the job:

SELECT DISTINCT ON (id) id, x, y FROM table