Answer
A LEFT JOIN keeps every row from the left input and matching rows from the right input. • Unmatched right-side columns become NULL. • More than one matching right row produces more than one result row. • Conditions placed in WHERE can accidentally remove preserved unmatched rows.
💡 SQL Example
SELECT c.name, o.order_id FROM customers c LEFT JOIN orders o ON o.customer_id = c.customer_id;
Result
Every customer, including customers without orders
⚡ Quick Revision
LEFT JOIN preserves all left-side rows.