Subquery vs JOIN vs EXISTS — Which One Should You Actually Use?

If you’ve ever written a SQL query and wondered “wait… is there a faster way to do this?” — you’re in the right place.

Welcome, curious learner! 👋

Whether you’re prepping for a data interview or just trying to write better SQL, this one will seriously level up your thinking.

Today we’re going to break down three ways to query related tables in SQL — and more importantly, why one can be painfully slow while another is lightning fast.

No scary jargon. Just simple explanations, real examples, and the kind of insight that makes interviewers nod approvingly. 😄

Let’s go!

🧠 The Golden Rule of SQL Performance

Before we dive in, burn this into your memory:

SQL engines love “Sets” (groups of data). They hate “Loops.”

That’s it. That’s the whole secret. Everything we discuss today comes back to this one idea.

🐢 Method 1: The Correlated Subquery (The Slow One)

What is it?

A Correlated Subquery is simply a query inside another query — where the inner query depends on each row from the outer query to run.