SQL Subqueries - Using a Query Inside Another Query

Introduction

Sometimes, solving a problem directly in SQL isn’t possible in a single step. You need to find something first — and use that result inside another query. That’s the power of subqueries. Also known as nested queries, subqueries allow one query to feed into another, like a conversation between clauses.

What is a Subquery?

A subquery is a SQL query inside another query. It’s usually enclosed in parentheses and can be used in SELECT, FROM, or WHERE clauses.

Types of Subqueries

• Scalar Subquery – returns a single value
• Row Subquery – returns one row with multiple columns
• Table Subquery – returns multiple rows and columns (used in FROM clause)

Sample Tables – students and results

CREATE TABLE students (
  roll_no INT PRIMARY KEY,
  name VARCHAR(50),
  class VARCHAR(10)
);

INSERT INTO students VALUES
(1, 'Aarav Sharma', '10A'),
(2, 'Diya Iyer', '9B'),
(3, 'Sneha Patil', '10A'),
(4, 'Mehul Agarwal', '8C');

CREATE TABLE results (
  roll_no INT,
  subject VARCHAR(30),
  marks INT
);

INSERT INTO results VALUES
(1, 'Maths', 85),
(2, 'Maths', 92),
(3, 'Maths', 78),
(4, 'Maths', 90);

1. Subquery in WHERE Clause

Let’s find students who scored more than the average marks in Maths:

SELECT name
FROM students
WHERE roll_no IN (
  SELECT roll_no
  FROM results
  WHERE subject = 'Maths'
  AND marks > (
    SELECT AVG(marks)
    FROM results
    WHERE subject = 'Maths'
  )
);

name
-------------
Diya Iyer
Mehul Agarwal

2. Subquery in SELECT Clause

Show each student's marks and how they compare to the average:

SELECT s.name, r.marks,
  (SELECT AVG(marks) FROM results WHERE subject = 'Maths') AS avg_marks
FROM students s
JOIN results r ON s.roll_no = r.roll_no
WHERE r.subject = 'Maths';

name           | marks | avg_marks
----------------+-------+-----------
Aarav Sharma   | 85    | 86.25
Diya Iyer      | 92    | 86.25
Sneha Patil    | 78    | 86.25
Mehul Agarwal  | 90    | 86.25

3. Subquery in FROM Clause

Let’s first calculate average marks per student, then filter only those with more than 80:

SELECT t.roll_no, t.avg_marks
FROM (
  SELECT roll_no, AVG(marks) AS avg_marks
  FROM results
  GROUP BY roll_no
) AS t
WHERE t.avg_marks > 80;

roll_no | avg_marks
--------+-----------
1       | 85.0
2       | 92.0
4       | 90.0

4. Correlated Subqueries

These refer to columns from the outer query. Example: Get students whose marks are higher than the class average.

SELECT name
FROM students s
WHERE EXISTS (
  SELECT 1
  FROM results r
  WHERE r.roll_no = s.roll_no
  AND r.marks > (
    SELECT AVG(marks)
    FROM results
  )
);

Best Practices

• Use subqueries for readability and step-wise logic.
• Try CTEs (Common Table Expressions) for complex nested queries.
• Avoid over-nesting — too many levels may impact performance.

Summary

Subqueries allow you to solve problems by breaking them into parts. Whether it's filtering, calculating, or comparing — using one query inside another lets you add intelligence and precision to your SQL logic.

What’s Next?

Next up: Common Table Expressions (CTEs) — an advanced but readable way to structure complex queries more cleanly than subqueries.