Explain why you should use 'IS NOT NULL' instead of 'ISNULL' in SQL queries.

In SQL, 'IS NULL' and 'IS NOT NULL' are comparison operators used to test whether a column contains a NULL value. They are part of the SQL syntax and are understood by all relational databases. 'ISNULL' (or 'IFNULL' in MySQL) is a function that returns a specified value when the first argument is NULL. Using 'ISNULL' as a comparison operator is a syntax error because the parser expects a function call, not a boolean comparison. Therefore, to filter rows where a column is not null, you should write 'WHERE column IS NOT NULL'. This ensures correct syntax, portability, and clarity.