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434. INNER JOIN vs LEFT JOIN

Compare INNER JOIN and LEFT JOIN and understand when each should be used.

📘 Lesson 434 of 728 ⏱ 13 min 🎯 Intermediate

INNER JOIN returns only matching rows, while LEFT JOIN returns all rows from the left table along with matching rows from the right table.

Choosing the correct join type directly affects which records appear in the result.

Use INNER JOIN when only customers with orders are required and LEFT JOIN when all customers must be included.

SQL 
SELECT
    c.CustomerName,
    o.OrderID
FROM Customers c
LEFT JOIN Orders o
    ON c.CustomerID = o.CustomerID;

This distinction is fundamental in reporting, reconciliation, customer analytics, and data-quality analysis.

  • INNER JOIN keeps only matching records.
  • LEFT JOIN keeps all left-table records.
  • LEFT JOIN can reveal missing relationships.
  • The choice depends on the business requirement.
  • Changing JOIN type can significantly change row counts.
What is the main difference between INNER JOIN and LEFT JOIN?

INNER JOIN returns matching rows only, while LEFT JOIN returns all left-table rows and matching right-table rows.

When should LEFT JOIN be used?

Use LEFT JOIN when all records from the primary left table must remain in the result, even when related data is missing.

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