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A LEFT JOIN keeps all invoices and customers in the result even if they have zero contacts. This ensures accurate counts and prevents missing rows when no matching contact exists.
While the exact problem may not always appear, similar SQL join and aggregation questions are common in FAANG-style interviews. They test understanding of relational data, joins, and counting with conditions.
The optimal approach uses SQL joins combined with aggregation. By joining Invoices with Contacts and matching contact_email with Customers.email, you can count total contacts and trusted contacts using conditional aggregation.
The problem is best solved using relational joins and GROUP BY aggregation in SQL. LEFT JOIN ensures customers with no contacts are still included, while conditional counting identifies trusted contacts.