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PostgreSQL LEFT JOIN

A LEFT JOIN returns all rows from the left table, plus matching rows from the right table. Rows in the left table without matches in the right table still appear, but with NULLs for right table columns.

Example

SELECT c.customer_id, c.first_name, o.order_id
FROM customers c
LEFT JOIN orders o ON c.customer_id = o.customer_id;

Best Practices

  • Use LEFT JOIN when you must keep all rows from the left table, regardless of a match.
  • Check for NULL values in the right table columns if you need to filter unmatched rows.

Key Takeaways

  • LEFT JOIN includes all rows from the left table.
  • Non-matching rows in the left table get NULL values for right table columns.
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