How should nulls be handled?

[jwise-mfg]

Discussion points so far:

• Is null an error condition? (see Describe failure responses #26)
• Is null the result when a system has no data Quality indicator and the data is bad (see Data Quality is not commonly available Metadata #33)
• Is null allowed? (see: the creator of JSON thinks null was a bad idea)

[aronsemle]

Are there cases where I'd do anything useful with a null?

Here is an example of removing them. I want to store some part of the namespace to SQL. Assume the table doesn't exist yet. I read an ElementID on the API to get the values out, and on the first read try and create the SQL table with the data. I have nulls in the data. What do I do? I can't create a column of type null. So I need to leave them out anyway, and update the table later if/when that attribute has a value.

I personally I'm not a fan of null, NaN, etc, but I lose this battle with my dev team all the time. The hard part is null doesn't have a data type and not all destination systems support.

[randy-armstrong]

The OPC UA rules:

Types are nullable or non-nullable.
Non-nullable types have a 'default value'.
Alternate representations of a "no value" are considered semantically equivalent (null and "" or null and []).
Metadata can be used to specify that a field in a structure can be "not specified" and what happens if not specified (i.e. nulls are allowed within a specific context - not as a general rule).

[graemewelton]

To your original points @jwise-mfg:

• Null is an error condition (e.g. communication failure, divided-by-zero, etc.). It is a valid indicator that there is no value, which is equally important to non-null data.
• Not necessarily, bad quality data can be non-null. In systems where Quality is not a concept, null values are a robust way to indicate an error condition.
• I am not concerned whether we call it null or undefined, but we do need at least one of these "bottom types" to indicate "no value". My preference is null.

Regarding type, I believe all our types should be nullable.

[randy-armstrong]

There are 2 contexts where null shows up:

1. The primary value
2. Fields in a structured value.

In OPC UA 1) is always nullable. The additional rules apply only to fields in structures.

Null is not necessarily