What is a subquery?

A subquery is a query written inside another SQL query. It is used to retrieve data that will be used by the main query.

Syntax

SELECT column_name
FROM table_name
WHERE column_name = (
    SELECT column_name
    FROM another_table
);

Example

Suppose you have an Employees table.

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To find employees who earn more than the average salary:

SELECT name, salary
FROM Employees
WHERE salary > (
    SELECT AVG(salary)
    FROM Employees
);

How it works

1. The inner query (subquery) calculates the average salary.
2. The outer query selects employees whose salary is greater than that value.

Types of Subqueries

1. Single-row subquery — returns one value
2. Multiple-row subquery — returns multiple values
3. Correlated subquery — depends on the outer query
4. Nested subquery — subquery inside another subquery

Advantages

• Simplifies complex queries
• Improves readability
• Helps perform calculations before the main query executes

Simple Definition

A subquery is a query inside another query used to help the main query get the required result.