How are NULL values handled in SQL?

In SQL, NULL values are handled using specific operators and functions designed to deal with such values.

In SQL, a NULL value is a value that is unknown or missing. It's important to note that NULL is not the same as zero or a field with spaces. A field with a NULL value is a field with no value. When performing operations on data, SQL treats NULL as an unknown value. Therefore, any arithmetic operation involving NULL always yields NULL. For example, NULL + 10 is NULL.

To handle NULL values, SQL provides several specific operators and functions. The IS NULL and IS NOT NULL operators are used to test for NULL values. For instance, to select all rows in a table where a certain column is NULL, you would use a query like "SELECT * FROM table WHERE column IS NULL".

Similarly, the IS NOT NULL operator is used to select rows where a certain column is not NULL. For example, "SELECT * FROM table WHERE column IS NOT NULL".

SQL also provides functions like COALESCE and NULLIF to handle NULL values. The COALESCE function returns the first non-NULL value in a list. For example, "SELECT COALESCE(NULL, 1, 2)" would return 1, as it's the first non-NULL value in the list.

The NULLIF function returns NULL if two specified values are equal and the first value if they are not. For example, "SELECT NULLIF(1, 1)" would return NULL, as the two values are equal.

In addition, SQL provides the NVL function (in Oracle) or ISNULL function (in SQL Server) that allows you to replace NULL values with a specified value. For example, "SELECT NVL(column, 0) FROM table" would return 0 for all NULL values in the 'column'.

In summary, handling NULL values in SQL involves understanding that they represent unknown or missing data, and using specific operators and functions to test for and manipulate these values.