Postgresql – ORDER BY that doesn’t contain repeating values

order-bypostgresqlselect

Relevant SQL Fiddle (postgres 9.2)

How do I state a SELECT .. ORDER BY .. that avoids having repeated values for a column?

That is, for rows of the column "rank" (1,3,3,1) how do I get a result so that rows of that column are ordered as (1,3,1,3), garuanteeing that a row with column value 1 is never follow by a row with column value 1, and similar for other rows values.

Best Answer

You cannot avoid in it in all cases (if more than 50% of the rows have the same value in the column.) One way to achieve a "shuffle" result, similar to what you want is by using window function:

WITH cte AS
  ( SELECT *,
           ROW_NUMBER() OVER (PARTITION BY colX) AS rn
    FROM tableX 
  )
SELECT *
FROM cte
ORDER BY rn, colX ;

The above will not avoid all cases though. If for example, the values in the column are 1,1,1,1,2,2,3, you'll get:

1,2,3,1,2,1,1

and not the (better):

1,2,1,3,1,2,1
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