Solving SQL Problems with Subqueries

Context

Subqueries are a common SQL technique for breaking a problem into smaller steps, especially when you need to compare rows against aggregated or filtered results. Interviewers often use them to test whether you can structure query logic clearly.

Question

Explain how to solve SQL problems involving subqueries. In your answer, cover:

1. What a subquery is and where it can appear in a SQL statement
2. Common use cases such as filtering, aggregation, and row comparison
3. The difference between scalar, correlated, and table subqueries
4. When a subquery is a good choice versus when a join or CTE may be clearer

Scope Guidance

Keep the discussion practical. The interviewer is looking for a clear explanation of how subqueries work, the main patterns you would use in real SQL problems, and the most common mistakes to avoid.