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Why does a LEFT OUTER JOIN sometimes return NULL values in the joined columns?

easy📝 Conceptual Q10 of 15
SQL - LEFT and RIGHT JOIN
Why does a LEFT OUTER JOIN sometimes return NULL values in the joined columns?
ABecause the query has syntax errors
BBecause there is no matching row in the right table for some left table rows
CBecause NULL values are replaced with zeros automatically
DBecause LEFT OUTER JOIN only returns rows with matches
Step-by-Step Solution
Solution:

  1. Step 1: Understand LEFT OUTER JOIN behavior

    It returns all rows from the left table and matches from the right table.

  2. Step 2: Explain NULL in joined columns

    If no matching row exists in the right table, columns from right table show NULL.

  3. Final Answer:

    Because there is no matching row in the right table for some left table rows -> Option B

  4. Quick Check:

    NULLs appear when no match in right table [OK]
Quick Trick: NULLs in outer join mean no matching row on that side [OK]
Common Mistakes:
MISTAKES
  • Assuming NULL means syntax error
  • Expecting automatic zero replacement
  • Thinking outer join excludes unmatched rows

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