In SQL, the NULL function is not specifically a built-in function. Instead, NULL is a special value that represents the absence of data or the unknown value. NULL is often used to indicate missing or undefined data in database columns.

To check for NULL values in SQL, you can use the IS NULL or IS NOT NULL operators. Here's an example:

sql
SELECT column1, column2
FROM table_name
WHERE column1 IS NULL;

In this example, the query selects the values from `column1` and `column2` from the table `table_name` where `column1` is NULL.

You can also use the COALESCE function in SQL to handle NULL values. The COALESCE function returns the first non-NULL value from a list of expressions. Here's an example:

sql
SELECT COALESCE(column1, 'N/A') AS column1_alias, column2
FROM table_name;

In this example, if `column1` is NULL, the COALESCE function replaces it with the value 'N/A'. The result is then displayed as `column1_alias`.

It's important to handle NULL values appropriately in SQL queries, as they can affect result sets and calculations. Understanding how to use the IS NULL operator and the COALESCE function can help you manage NULL values effectively in your SQL statements.