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Why is it important that the subquery used with the IN operator returns a single column?

hard📝 Conceptual Q10 of 15
SQL - Subqueries
Why is it important that the subquery used with the IN operator returns a single column?
ABecause subqueries with multiple columns cause the main query to run faster
BBecause subqueries with multiple columns are automatically converted to joins
CBecause IN compares a single value to a list of single values
DBecause IN operator can only be used with numeric columns
Step-by-Step Solution
Solution:

  1. Step 1: Understand IN operator comparison

    IN compares one value from main query to a list of values from subquery.

  2. Step 2: Explain why single column is needed

    The subquery must return a single column to form a valid list of values for comparison.

  3. Final Answer:

    Because IN compares a single value to a list of single values -> Option C

  4. Quick Check:

    IN needs single-column subquery results [OK]
Quick Trick: Subquery with IN must return one column only [OK]
Common Mistakes:
MISTAKES
  • Assuming multiple columns are allowed
  • Thinking IN only works with numbers
  • Confusing IN with JOIN behavior

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