jay jay - 4 years ago 410
SQL Question

Postgres Rails Select Distinct with Order

This seems harder than it should be:

I want to be able to sort a table by it's copy_count, then select only events with a unique title, and limit that query to the first 99.

Event.order("copy_count DESC").select("DISTINCT ON (events.title) *").limit(99)

This throws an error:

ActiveRecord::StatementInvalid: PG::Error: ERROR: SELECT DISTINCT ON expressions must match initial ORDER BY expressions

Which suggest I need to add the copy_count to the DISTINCT ON, but this would also only pull back unique copy_count records which could be the same!

Note: the order by the copy_count MUST happen first.


Answer Source

For the pure SQL it will look like:

FROM (SELECT DISTINCT ON (events.title) *
      FROM events
      ORDER BY events.title, events.copy_count DESC) top_titles
ORDER BY events.copy_count DESC

But i don't know, how to write it in RoR.

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