Hazards of SQL SELECT DISTINCT

Although SQL’s DISTINCT keyword creates a list of unique values like usernames or route steps, it’s dangerous in practice. It can:

  • mask actual duplicates – which can mean entered data might be recorded under the wrong entry (or the right entry, or some under each, or duplicated in both),
  • and it can mask false duplicates caused by a malformed query that doesn’t return a unique natural key because not all columns are included or displayed.

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As a quick check to verify there’s no deeper investigation needed before removing the keyword, run the query before and after and compare the rowcount. Deeper investigation typically looks like concatenating some columns, restructuring the query, or research and repairing the dupe.

 

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