Important Concept: EXISTS vs JOIN

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Understanding when to use EXISTS and when to use JOIN is very important in SQL.

1. Use EXISTS when you only care about “Existence” (at least once)

WHERE EXISTS (
    SELECT 1 
    FROM FactSurveyResponse SR 
    WHERE SR.CustomerKey = C.CustomerKey
)

Result: Each customer appears only once, even if they have many survey responses.

2. Use JOIN when you need data from the other table

LEFT JOIN FactSurveyResponse SR 
    ON C.CustomerKey = SR.CustomerKey
WHERE SR.CustomerKey IS NOT NULL

Result: Can return duplicate rows if a customer responded to multiple surveys.

Key Takeaway

  • EXISTS / NOT EXISTS → Best for checking “Does this exist at least once?”
  • JOIN → Best when you want to bring columns from another table
  • JOIN can create duplicate rows if there are multiple matches

This concept appears frequently in healthcare analytics — for example:

  • Members who had at least one denied claim
  • Products that were never sold
  • Patients who have specific conditions
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