I actually wonder if many individual cops use "null" (the ordinary word) as a shorthand for "plate not applicable," "not identified", "missing" etc - semantically the same thing as NULL in programming, but then in this case it wouldn't be a database NULL error. In theory the same thing could happen if someone had a DEALER vanity plate. (Though that choice might be rejected for obliquely referencing drugs.)

There are so SO many databases out in the wild that were built by people with little regard for building them correctly - or they simply not programmers/DBA's in the first place, but their boss told them to just make it happen.

This is genuinely terrifying to think about. All the ones that are really important are correct, right...? Like medical systems and financial systems and legal and..

If there are multiple systems involved, one of them produces a null and the other takes that as a key to create a record, but somewhere in between it gets stringified, then the string null might be accepted by a system using string keys.

And here we have identified that the system doing that uses PHP and its associative arrays.

I'd guess that something somewhere has got its sanitization wrong. They tested it against

    Robert'); DROP TABLE Students;--

And see that it now provides

    'Robert DROP TABLE Students'

Problem solved! And then later somebody else comes along, ignorant of the sanitization step, and provides

    NULL

But the code strips special characters and adds quotes, so they've actually inserted:

    'NULL'

It'd be sufficient if a system involved somewhere in the process converted null values to strings. There's innumerable ways, but here's a simple one in Java:

    final String myNullString = "" + null;
    System.out.println(myNullString);

They all do. But these are seperate databases that get translated to text at some point and then converted to Null

CREATE TABLE orders ( id INT PRIMARY KEY, customer_id INT NULL, FOREIGN KEY (customer_id) REFERENCES customers(id) );

vs

CREATE TABLE orders ( id INT PRIMARY KEY, customer_id INT NOT NULL, FOREIGN KEY (customer_id) REFERENCES customers(id) );

The simple explanation is most of these are Excel VLOOKUPs gone wrong.

The real question is why anything in the STRING is getting interpreted at all. In databases ‘null’ is not the same thing as NULL.

Wait, you're telling me "NULL" is not equal to NULL?

no value is equal to NULL, including NULL