IN clause

Simple IN clause

To get records having any of the given ids

select *
from products
where id in (1,8,3)

The query above is equal to

select *
from products
where id = 1
   or id = 8
   or id = 3

Using IN clause with a subquery

SELECT *
FROM customers
WHERE id IN (
    SELECT DISTINCT customer_id
    FROM orders
);

The above will give you all the customers that have orders in the system.