COALESCE is a function that returns the first argument you give it that isn’t NULL

COALESCE(NULL, 1, 2) => 1
COALESCE(NULL, NULL, NULL) => NULL
COALESCE(4, NULL, 2) => 4

2 ways you might want to use COALESCE in practice:

1. Set a default value:

In this table, a NULL discount means there’s no discount, so we use COALESCE to set the default to O:

SELECT name,
       price - COALESCE(discount, 0) as net_price
FROM products

products:

query output:

2. Use data from 2 (or more!) different columns

This query gets the best guess at a customer’s state:

SELECT customer,
   COALESCE(mailing_state, billing_state, ip_address_state) AS state
FROM addresses

(Mailing address, most accurate. If not, try billing address. As a last resort, use their IP address)