Explain how ColdFusion handles null values within query result sets.

ColdFusion represents null values retrieved from database query result sets as the special value `cf_null`. This is distinct from other programming languages where null might be represented as `NULL`, `None`, or similar. `cf_null` is a ColdFusion-specific object that signifies the absence of a value in a database field. When a query returns a null value from a database column, ColdFusion automatically converts that database null into a `cf_null` object within the resulting structure (typically an array or a struct). This conversion happens transparently; you don't need to explicitly request it.

ColdFusion provides several ways to check for and handle `cf_null` values. The most common is using the `is cf_null` test. This operator evaluates to `true` if the variable on its left is a `cf_null` object, and `false` otherwise. For example, `<cfif myVariable is cf_null> ... </cfif>` will execute the code block if `myVariable` contains a null value. Another method is using the `not` operator in conjunction with `is cf_null`. `<cfif not myVariable is cf_null> ... </cfif>` executes if `myVariable` does *not* contain a null value.

Directly using a `cf_null` variable in arithmetic operations or string concatenation will typically result in the entire expression evaluating to `cf_null`. This is because ColdFusion treats `cf_null` as a 'null' value in these contexts. For instance, `1 + cf_null` results in `cf_null`. However, ColdFusion provides functions to handle these situations gracefully. The `IsNull()` function returns `true` if its argument is `cf_null`, and `false` otherwise. This allows you to conditionally perform operations only when a value is present. The `Coalesce()` function returns the first non-`cf_null` argument. This is useful for providing default values when a field might be null. For example, `Coalesce(myField, 'Default Value')` will return the value of `myField` if it's not `cf_null`; otherwise, it will return 'Default Value'.

When displaying `cf_null` values in a web page, ColdFusion will typically render them as an empty string by default. This behavior can be modified using the `cfoutput` tag's `null` attribute. Setting `null=""` will display an empty string; `null="N/A"` will display 'N/A', and so on. If the `null` attribute is not specified, ColdFusion will render `cf_null` as an empty string. It's important to note that `cf_null` is *not* the same as an empty string (`