SQL Subqueries Practice Questions

A subquery is a SELECT nested inside another statement, used when one query needs the answer to a smaller query first. A scalar subquery returns a single value and can sit in the SELECT list or a comparison, while a correlated subquery references the outer row and is re-evaluated for each of them. These questions build the judgement for when a subquery is the clearest tool and when a join or CTE would read better and run faster.

What you will practice

• •Use a scalar subquery to compare each row against an overall value such as the company average
• •Write correlated subqueries with EXISTS and NOT EXISTS to test for related rows
• •Filter with IN versus EXISTS and understand the NULL trap in NOT IN
• •Put a subquery in the FROM clause as a derived table to aggregate twice
• •Recognise when to rewrite a nested subquery as a CTE or a join for readability

15 of 16 questions in this topic

• Tracks Above Average Pricebeginner
• Correlated Subquery: Above Genre Averageintermediate
• EXISTS: Find Customers with Large Purchasesbeginner
• NOT EXISTS: Artists Without Albumsbeginner
• Scalar Subquery in SELECTbeginner
• Nested Aggregation: Average of Totalsintermediate
• Calculate Percentage of Totalintermediate
• Catalog Snapshot Reportintermediate
• Customers Active in Multiple Yearsintermediate
• Top 3 Spenders per Countryintermediate
• Genres With Above-Average Pricingintermediate
• First-Purchase Cohort Count by Monthintermediate
• Invoices Above the Overall Average (Scalar Subquery)intermediate
• Tracks That Have Never Been Purchased (Anti-Join)intermediate
• Tracks Priced Above the Catalog Average (Scalar Subquery)intermediate