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What is the most common cause of inflated totals after a SQL join?

  1. A Joining on a non-unique key, which multiplies rows
  2. B Using the wrong aggregate function
  3. C Missing an index
  4. D Selecting too many columns
Answer

Joining on a non-unique key, which multiplies rows

Always compare the row count before and after a join. This single check catches a large share of analysis errors.

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