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From my understanding, primary keys are unique by definition. If that's true, would one ever want to SELECT DISTINCT on the primary key? Is this ever practical/useful?

  • Yes it is unique for the record so without joining to other tables, DISTINCT wouldn't matter. However, in a one to many relationship, or when joining to a table with unnormalized data, it may be beneficial. – scsimon Jun 22 '18 at 13:45
  • You don't have to use DISTINCT in a PRIMARY KEY columns since its already UNIQUE. There is also a performance impact when using DISTINCT. – Edgar Allan Bayron Jun 22 '18 at 14:06
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If you join with another m:n table that duplicates the PK value in your result set, then it would be useful.

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  • (Or 1:n for that matter) – Koen D Jun 22 '18 at 13:46
  • Note that multi-column distinct groups are more efficiently queried using the GROUP BY clause. – Elaskanator Aug 15 '18 at 16:20

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