SQL SUBQUERY Note 1

🔍 What is a Subquery?

A subquery (also called an inner query or nested query) is a query placed inside another SQL query — used to return data that the outer query depends on.

It is enclosed within:

• () parentheses
• Often used in WHERE, FROM, or SELECT clauses

🔍 Why Use Subqueries?

• To break complex problems into smaller pieces
• To avoid joins in some cases
• To filter, transform, or compare data using values from another query

🔍 Types of Subqueries?

• Independent (Non-correlated) Subquery
• Correlated Subquery

🔎 How to Identify Each?

Independent Subquery

• It can run alone.
• It does not refer to any column from the outer query.

💡 Key sign: No column inside the subquery refers to the outer query’s table.

Correlated Subquery

• It depends on values from the outer query.
• It must reference a column from the outer query.
• It cannot run on its own.

💡 Key sign: You’ll see a column from the outer query inside the subquery.