-- ISNULL() function replaces NULL with a specified value. -- The ISNULL() function returns the replacement if the expression evaluates to NULL. -- Before returning a value, it implicitly converts the ...

Is Null and IsNull () both find null values, but you won’t use them in the same way. You would use Is Null and Is Not Null in query expressions and SQL WHERE clauses.

If you have calculations that need to vary based on optional data, the left join method with isnull, case, etc. works well.