What is the difference between INNER JOIN and LEFT JOIN?
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In SQL, INNER JOIN and LEFT JOIN are both used to combine rows from two tables, but they differ in how they handle unmatched rows.
🔑 INNER JOIN
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Returns only rows where there is a match in both tables.
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If no matching row exists in either table, that row is excluded.
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✅ Result: Only employees who are assigned to an existing department.
🔑 LEFT JOIN (LEFT OUTER JOIN)
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Returns all rows from the left table, plus matching rows from the right table.
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If no match is found, columns from the right table are filled with NULLs.
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✅ Result: All employees are listed, including those without a department (dept_name = NULL).
⚡ Key Differences
| Feature | INNER JOIN | LEFT JOIN (OUTER) |
|---|---|---|
| Matching Rows Only | ✅ Yes, only matches | ❌ No, includes unmatched rows too |
| Unmatched Rows | Excluded | Left table rows kept, right = NULL |
| Result Size | Smaller or equal to LEFT JOIN | Greater or equal to INNER JOIN |
| Use Case | Find records with relationships | Find all from left, even without match |
👉 Summary:
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INNER JOIN = Intersection → Only rows with matches in both tables.
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LEFT JOIN = Preservation → All rows from left + matches (or NULLs if no match).
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