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What is the main difference between a FULL OUTER JOIN and a LEFT JOIN in PostgreSQL?

easy📝 Conceptual Q1 of 15
PostgreSQL - Joins in PostgreSQL
What is the main difference between a FULL OUTER JOIN and a LEFT JOIN in PostgreSQL?
AFULL OUTER JOIN returns rows with NULLs removed, LEFT JOIN returns rows with NULLs included
BFULL OUTER JOIN returns only matching rows, LEFT JOIN returns all rows from both tables
CFULL OUTER JOIN returns only rows from the right table, LEFT JOIN returns only rows from the left table
DFULL OUTER JOIN returns all rows from both tables, LEFT JOIN returns all rows from the left table only
Step-by-Step Solution
Solution:
  1. Step 1: Understand LEFT JOIN behavior

    LEFT JOIN returns all rows from the left table and matching rows from the right table. If no match, right side columns are NULL.
  2. Step 2: Understand FULL OUTER JOIN behavior

    FULL OUTER JOIN returns all rows from both tables. If no match, NULLs fill missing side columns.
  3. Final Answer:

    FULL OUTER JOIN returns all rows from both tables, LEFT JOIN returns all rows from the left table only -> Option D
  4. Quick Check:

    JOIN type difference = A [OK]
Quick Trick: FULL OUTER JOIN keeps all rows from both tables [OK]
Common Mistakes:
  • Confusing FULL OUTER JOIN with INNER JOIN
  • Thinking LEFT JOIN returns rows from both tables
  • Assuming FULL OUTER JOIN excludes NULLs

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