EXISTS usage and examples

Overview

EXISTS tests only whether a subquery returns at least one row; the selected values are irrelevant. A correlated subquery refers to the current outer row and is useful for checking whether related records exist. Neither EXISTS nor IN is universally faster, so choose clear semantics and verify with indexes and an execution plan.

Basic syntax

SELECT * FROM customers c WHERE EXISTS (SELECT 1 FROM orders o WHERE o.customer_id = c.id);

Examples

EXISTS example

Check each customer and keep only those with at least one related row in orders.

Sample table

Table：customers

customers
id	name	city	status	phone	nickname
101	Alice	New York	active	138-0000-0001	NULL
102	Bob	Seattle	active	NULL	Bobby
103	Cindy	Austin	inactive	138-0000-0003	NULL

Table：orders

orders
id	customer_id	product_id	name	department	amount	status	created_at	discount
1	101	201	Alice	Sales	1200	paid	2026-01-03	0
2	102	202	Bob	IT	900	pending	2026-01-08	50
3	101	201	Cindy	Sales	1500	paid	2026-02-02	100
4	104	203	David	HR	700	cancelled	2026-02-15	0

Run result

result
id	name	city	status	phone	nickname
101	Alice	New York	active	138-0000-0001	NULL
102	Bob	Seattle	active	NULL	Bobby

Common issues

Does EXISTS care which columns the subquery returns?

No. EXISTS only checks whether the subquery returns at least one row.

When is EXISTS commonly used?

It is commonly used to check whether related records exist for a main-table row.

Is EXISTS always faster than IN?

No. Performance depends on data size, indexes, and the optimizer. Clarity comes first.

Keep reading

IN WHERE

EXISTS