Which statement correctly describes a left join in SQL?

• A. Inner join returns only matching keys.
• B. It returns all rows from the left table and matching rows from the right; nonmatches on the right are null.
• C. It returns all rows from the right table.
• D. It returns the Cartesian product.

Answer: (B)

A left join keeps every row from the left table and brings in matching data from the right table. If there’s no corresponding row in the right table, the right-side columns appear as NULL for that left row. This is exactly what makes left joins useful when you want to preserve all records from the left side regardless of whether there’s related data on the right.

For example, if you have a table of customers (left) and a table of orders (right), a left join on customer_id will show every customer, and the order fields will contain data where that customer has orders, or NULL if the customer hasn’t placed any orders.

This differs from an inner join, which would only return customers who have matching orders, omitting those without orders. It also differs from a right join, which would preserve all rows from the right table instead of the left. And a Cartesian product would pair every row of the left table with every row of the right table, which is not what a left join does.