SQL Academy · Lesson

EXISTS vs JOIN Performance

Choose the faster pattern.

Why Performance Matters Here

When you need to check whether related rows exist in another table, SQL gives you several tools: EXISTS, IN, and JOIN. Each produces correct results, but they can perform very differently depending on your data size, indexes, and database engine.

In this lesson you will learn how each approach works under the hood and when to reach for which one.

Sample Tables

We will use two tables throughout this lesson: customers and orders. A customer may have zero or many orders. This is a classic one-to-many relationship perfect for testing EXISTS vs JOIN patterns.

CREATE TABLE customers (
  id   SERIAL PRIMARY KEY,
  name VARCHAR(100)
);

CREATE TABLE orders (
  id          SERIAL PRIMARY KEY,
  customer_id INT REFERENCES customers(id),
  total       NUMERIC(10,2)
);

INSERT INTO customers (name) VALUES
  ('Alice'), ('Bob'), ('Carol'), ('Dave');

INSERT INTO orders (customer_id, total) VALUES
  (1, 120.00), (1, 85.50), (3, 200.00);

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