SQL Joins Explained

• INNER JOIN
  • Only rows that match in both tables
  • Drops unmatched rows on either side
• LEFT JOIN
  • All rows from the left table
  • NULLs where the right table has no match
  • Typical use: all users, even those without orders
• RIGHT JOIN
  • Mirror image of LEFT JOIN
  • Rarely needed; rewrite as LEFT JOIN for readability
• FULL OUTER JOIN
  • All rows from both tables
  • NULLs on either side where no match exists
• CROSS JOIN
  • Cartesian product: every row paired with every row
  • Result size = rows(A) × rows(B)
• Common Mistakes
  • Missing ON condition produces an accidental cartesian product
  • Duplicate rows usually mean an unaggregated one-to-many
  • NULL never equals NULL in join conditions