Because of some errors generated by the core when mysql 5.6.6+ (i think) is in it's default sql mode, joomla modifies the sql mode to none (see, for instance, https://github.com/joomla/joomla-cms/blob/4.0-dev/libraries/joomla/database/driver/mysqli.php#L187).

You can also see this was done for PDO Mysql driver here https://github.com/joomla/joomla-cms/pull/9485/files

See, for more info about the sql modes (ex: it's default values):

• http://dev.mysql.com/doc/refman/5.7/en/sql-mode.html
• http://dev.mysql.com/doc/refman/5.6/en/sql-mode.html

The problem with this no sql mode, is that it silently ignore some db consistency problems.

So, as talked when the problem was discovered, should 4.0 only allow mysql strict mode?
And if so, which parameters should joomla use for this sql mode?

anyway this of course will be a B/C break, since core and extensions which don't have the tables/queries in accordance to the sql mode will give errors.