In SQL Server, you can use the ISNULL function to replace NULL values. In MariaDB, you can use the IFNULL function.
SQL Server:
-- Replace NULL with N/A string SELECT ISNULL(NULL, 'N/A'); # N/A
MariaDB:
-- Replace NULL with N/A string SELECT IFNULL(NULL, 'N/A'); # N/A
For more information, see SQL Server to MariaDB Migration.