The built-in function coalesce in MySQL is great for outer joins where the return or expected answer can be null. Coalesce can be used anytime a null could be expected. It accepts a comma-delimited list of values or column names and returns the 1st in the list that isn’t null.
This example below will return the value of column_1 if it’s not null, otherwise, it will return 1.
select coalesce(column_1, 1) as column_1