I'd imagine that the warning is refering to the NULL being placed in a NOT NULL column.
You can view the warning message by running.
SHOW WARNINGS;
Personally I think it's a bit of an oversight that it doesn't actually explicitly show you the warning in the way it does with errors. If it's important enough to warn you about it then it's important enough to show it in full.
Back to the NULL, I think it's just a case of knowing what it does and coding to deal with it. If you think it's a big enough issue that you wouldn't want to use MySQL because of this then thats fine and to some degree I'd understand that point of view.
Andrew Gilfrin
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