⬅️ LEFT JOIN in SQL

Last Updated: 7th January 2026

The LEFT JOIN returns all rows from the left table and matching rows from the right table.

  • If there is no match, columns from the right table become NULL.
  • Left table → always preserved
  • Right table → included only if matched

Hinglish Tip 🗣: Left table ka poora data chahiye, match ho ya na ho → LEFT JOIN.

🧾 Basic Syntax

SELECT columns
FROM table1
LEFT JOIN table2
ON table1.common_column = table2.common_column;

📄 Example Tables (Mental Model)

customers (Left Table)

customer_idname
1Rahul
2Anjali
3Aman

orders (Right Table)

order_idcustomer_id
1011
1023

📄 LEFT JOIN Example

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

✅ Result

nameorder_id
Rahul101
AnjaliNULL
Aman102

Explanation:

  • Rahul → has order → shown
  • Anjali → no order → still shown (NULL)
  • Aman → has order → shown

🚫 WHERE vs ON (Very Important)

❌ This breaks LEFT JOIN logic:

LEFT JOIN orders o
ON c.customer_id = o.customer_id
WHERE o.order_id IS NOT NULL;

✔ Correct way (when filtering right table):

LEFT JOIN orders o
ON c.customer_id = o.customer_id
AND o.order_id IS NOT NULL;

Reason:

  • WHERE runs after JOIN
  • It can turn LEFT JOIN into INNER JOIN accidentally

🔄 LEFT JOIN + WHERE

SELECT c.name
FROM customers c
LEFT JOIN orders o
ON c.customer_id = o.customer_id
WHERE o.order_id IS NULL;

✔ Finds customers with no orders

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