The COALESCE function in SQL is useful when you want to take a value but use a default if it’s null. COALESCE returns the first non-null value from a list of arguments.

For example:

SELECT symbol FROM currency c
WHERE c.currency_id = COALESCE(payment.currency_id, 1)

In this query, COALESCE(payment.currency_id, 1) returns payment.currency_id if it’s not null; otherwise, it returns 1. This ensures you always get a valid currency ID, avoiding potential null value issues.