How to Write Complex Queries with Nested Subqueries in SQL?

Nested subqueries in SQL are queries placed inside another query, allowing you to filter, aggregate, or manipulate data more efficiently. They help solve complex problems by breaking them into manageable parts.

Databases power almost every modern application—from eCommerce to finance. But as data grows, queries become complex. Nested subqueries in SQL give developers the ability to handle multi-layered data problems with precision, without writing overly complicated joins.

Quick Summary of Nested Subqueries in SQL

• Nested Subqueries → Queries inside queries for advanced filtering.
• Use Cases → Reporting, analytics, real-time dashboards.
• Advantages → Simplifies logic, reduces repetitive code.
• Limitations → Performance issues with large datasets.

What are Nested Subqueries in SQL?

Nested subqueries, also called inner queries or subqueries, are SQL queries placed inside another query. They allow you to perform intermediate calculations or filtering before the main query executes. Nested subqueries can return single values, lists of values, or entire tables depending on the context.

Example:

-- Find employees with salaries higher than the average salary
SELECT name, salary
FROM employees
WHERE salary > (SELECT AVG(salary) FROM employees);

Output:

name	salary
John Doe	75000
Jane Smith	80000

When Should You Use Nested Subqueries?

Nested subqueries are useful in scenarios such as:

• Filtering aggregated results: Finding records based on calculated averages, sums, or counts.
• Conditional joins: Applying conditions that depend on another table without explicitly writing multiple JOINs.
• Multi-step calculations: Breaking complex logic into smaller, manageable steps.
• Dynamic selection: Returning a set of values that determine the main query’s result.

Types of Nested Subqueries in SQL

1. Single-row subquery: Returns a single value used in comparison operators like =, <, or >.
2. Multi-row subquery: Returns multiple rows, often used with IN, ANY, or ALL operators.
3. Correlated subquery: References columns from the outer query and executes once per outer query row.

Example: Writing a Nested Subquery in SQL

Scenario: Retrieve orders where the total order amount is higher than the average order amount.

SELECT order_id, customer_id, total_amount
FROM orders
WHERE total_amount > (SELECT AVG(total_amount) FROM orders);

Output:

order_id	customer_id	total_amount
101	C001	1200
105	C003	1500
110	C002	1300

Practical Uses of Nested Subqueries in SQL

Nested subqueries are widely used in real-world applications to handle complex data analysis, reporting, and dynamic filtering. Here are some notable examples:

1. Google – Data Analysis for Search Rankings
Google uses nested subqueries internally to analyze large datasets for ranking algorithms and indexing results. For example, they may identify pages with traffic above the average in a given category:

SELECT page_id, page_title, traffic
FROM web_pages
WHERE traffic > (SELECT AVG(traffic) FROM web_pages WHERE category = 'Technology');

Output:

page_id	page_title	traffic
1001	AI Trends 2025	95000
1005	Cloud Computing	87000

This allows analysts to focus on high-performing pages in each category efficiently.

2. Amazon – Product Filtering for Recommendations
Amazon uses nested subqueries to filter products based on user behavior or aggregated statistics. For example, finding products priced above the average in a category:

SELECT product_id, product_name, price
FROM products
WHERE price > (SELECT AVG(price) FROM products WHERE category = 'Electronics');

Output:

product_id	product_name	price
2001	Noise Cancelling Headphones	250
2003	Smartwatch Pro	300

This helps the platform suggest premium items to users looking for high-value products.

3. Airbnb – Advanced Booking Reports
Airbnb uses nested subqueries for analytics such as identifying hosts with above-average booking counts or revenue:

SELECT host_id, COUNT(booking_id) AS total_bookings
FROM bookings
WHERE host_id IN (
    SELECT host_id
    FROM bookings
    GROUP BY host_id
    HAVING COUNT(booking_id) > (SELECT AVG(counts) FROM (SELECT COUNT(booking_id) AS counts FROM bookings GROUP BY host_id) AS avg_table)
)
GROUP BY host_id;

Output:

host_id	total_bookings
H001	120
H005	110

This allows Airbnb to target top-performing hosts for promotions or incentive programs.

4. Financial Institutions – Risk Analysis
Banks and investment firms use nested subqueries to flag transactions exceeding the average of a customer segment, improving fraud detection and risk management:

SELECT transaction_id, customer_id, amount
FROM transactions
WHERE amount > (SELECT AVG(amount) FROM transactions WHERE segment = 'Premium');

Output:

transaction_id	customer_id	amount
T1001	C501	15000
T1005	C508	20000

This ensures analysts can quickly focus on high-risk transactions.

Pros & Cons of Nested Subqueries in SQL

Aspect	Pros	Cons
Readability	Easy to break down complex logic	Becomes confusing with deep nesting
Performance	Good for small/medium datasets	Slower for very large data
Flexibility	Works across SQL dialects	Sometimes joins are more efficient

Interview Questions: Nested Subqueries

Nested subqueries in SQL can be a bit puzzling, can’t they? Folks often find themselves scratching their heads over these. To shed some light on this, I’ve compiled a list of common questions folks are asking online about nested subqueries. Here’s a rundown:

1. What exactly is a nested subquery, and why do we need it?
   A nested subquery is an SQL query within another SQL query. Think of it as a query layer cake. They’re mainly used when you need to perform complex filtering or calculations that can’t be easily done in a single query.
2. Can you use a nested subquery inside the SELECT clause?
   You can use a nested subquery within the SELECT clause to derive calculated values. For example:
   

     SELECT name, (SELECT COUNT(*) FROM orders WHERE customers.id = orders.customer_id) AS order_count 
     FROM customers;
   

3. How do nested subqueries affect performance?
   They can sometimes impact performance, because each subquery may mean additional computation and resources. But hey, if optimized correctly, the effect can be minimized.
4. Is there a limit to how deep you can nest subqueries?
   In practice, you can nest quite deeply, though readability and performance issues might arise long before any system-imposed limits.
5. How can I optimize a query with nested subqueries?
   Optimize by ensuring your subquery logic is as efficient as possible, and make use of indexing on columns involved in the subqueries.
6. Can alternatives like JOINs replace nested subqueries?
   Yup! Sometimes using JOINs can be more efficient and clearer than nesting subqueries, depending on what you’re trying to achieve.
7. Does a nested subquery always return a single value?
   No, it doesn’t have to. While a scalar subquery returns a single value, others can return entire sets of data depending on what’s being queried.
8. Can you nest subqueries in an UPDATE statement?
   Yes, you can! Subqueries in UPDATE statements can help set values based on complex conditions.
   

There’s always so much to learn about nested subqueries, but a little practice makes them easier to grasp. Over time, they won’t seem so nested in mystery!

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Conclusion

Learning ‘Nested Subqueries in SQL’ enhances your querying prowess, making data retrieval more efficient and targeted. By mastering these subqueries, you sharpen your problem-solving abilities and gain a deep sense of satisfaction. Why not give it a go? Check out more programming languages like Java and Python on Newtum.
– Download our free SQL cheat sheet to master queries faster.

Edited and Compiled by

This article was compiled and edited by @rasikadeshpande, who has over 4 years of experience in writing. She’s passionate about helping beginners understand technical topics in a more interactive way.

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