What is note-level warning in MySQL? - mysql

Question

Okay, I understand what are errors and warnings in the context of MySQL. But what's the need of note-level warning? I have already searched the MySQL documentation but didn't find anything relevant. It would be better if someone could shed some light on the what are they and why they are useful.
mysql> create database if not exists city;
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> show warnings
-> ;
+-------+------+------------------------------------------------+
| Level | Code | Message |
+-------+------+------------------------------------------------+
| Note | 1007 | Can't create database 'city'; database exists |
+-------+------+------------------------------------------------+
1 row in set (0.00 sec)

Answer 1

I've always considered Note to be like an "FYI": something happened, or didn't, that may be of interest. The closest definition I can find in the docs is:
... events that do not affect the integrity of the reload operation
which is from the sql_notes server variable, one perhaps not often used outside of mysqldump.
Trawling through the MySQL source code, looks like Sql_Condition::SL_NOTE annotates warnings of this level. There are a few, but they are mostly as you'd expect for non-impactful information:
Event already exists
Table already exists
Query '%s' rewritten to '%s' by a query rewrite plugin
Password set
Sadly, I would have expected the code docblock to give a little more information about them, but it doesn't:
class Sql_condition {
public:
/**
Enumeration value describing the severity of the condition.
*/
enum enum_severity_level { SL_NOTE, SL_WARNING, SL_ERROR, SEVERITY_END };
This might warrant a documentation bug report to MySQL team.
Interestingly, MariaDB has this to say:
A note is different to a warning in that it only appears if the sql_notes variable is set to 1 (the default), and is not converted to an error if strict mode is enabled.
My takeaway from that, in Maria and possibly by extension MySQL: notes are warnings, but ones that can be ignored because no data-loss or side-effect is described.