I started by joining transactions to customers which was fine, but I kept second-guessing whether to use SUM or COUNT and fumbled explaining it out loud.
Start by clarifying the schema and the definition of 'total sales' (e.g., sum of transaction amounts). Then write a SQL query that joins transactions to customers, groups by customer, sums the sales, and orders descending. If the interviewer expects handling of multiple tables, explain how books and authors relate but are not needed for this aggregation.
Pro tip: Mention that you would confirm whether 'unique customers' means distinct customer IDs or distinct customer names, and whether to include customers with zero sales. This shows attention to data semantics and business context.
Ask about the columns in each table, especially the foreign keys linking transactions to customers, and the definition of 'total sales' (e.g., sum of amount). Confirm if customers with no transactions should be included.
Determine that only transactions and customers are needed for this query. Explain that books and authors are not required unless sales need to be attributed to authors or books.
Construct a SQL query that joins transactions to customers on customer_id, groups by customer identifier, sums the transaction amount, and orders by the sum descending.
Discuss handling of NULLs, customers with no transactions (using LEFT JOIN), and indexing on foreign keys for performance. Mention that ordering by an alias is acceptable in most SQL dialects.
Walk through the query step by step, explaining each clause and how it satisfies the requirements. Be prepared to modify if the interviewer adds constraints.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.