[Solved] Identify the mistake in this query intended to find unmatched records:SELECT c.id FROM customers c LEFT JOIN orders o ON c.id = o.customer_id WHERE c.id IS NULL; | SQL

SQL - LEFT and RIGHT JOIN

Identify the mistake in this query intended to find unmatched records:

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

AThe query should use INNER JOIN instead of LEFT JOIN

BThe JOIN condition is incorrect; it should join on order_id

CThe WHERE clause should check for NULL in the right table's key, not the left table's

DThe SELECT clause should include columns from the orders table

Step-by-Step Solution

Solution:

Step 1: LEFT JOIN returns all left rows
Rows from customers (left) are always present.
Step 2: WHERE c.id IS NULL
This condition filters out all rows because c.id is never NULL in the left table.
Step 3: Correct NULL check
To find unmatched customers, check for NULL in the right table's key (o.customer_id).
Final Answer:
The WHERE clause should check for NULL in the right table's key, not the left table's -> Option C
Quick Check:
Check NULL on right table key after LEFT JOIN to find unmatched rows [OK]

Quick Trick: Check NULL on right table key, not left table [OK]

Common Mistakes:

MISTAKES

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