IFNA to LEFT JOIN and ISNULL

#N/A in a lookup is a row that did not match. LEFT JOIN + ISNULL is the whole translation.

SELECT t.customer_id, ISNULL(c.first_name, 'Unknown') AS first_name
FROM Tickets t
LEFT JOIN Customers c ON c.customer_id = t.customer_id;

An INNER JOIN drops the miss. That is not IFNA — that is a filter.

Canonical translation

SELECT t.customer_id, ISNULL(c.first_name, 'Unknown') AS first_name
FROM Tickets t
LEFT JOIN Customers c ON c.customer_id = t.customer_id;