the db fields like that:
id not null
username not null
email not null
province default null
city default null
age default null
when I execute
$user->update(['province'=>'aa', 'city'=>'bb', 'age'=>22])
all is fine.... But, after that, if I execute
$user->update(['province'=>'NY'])
then, the fields of city and age will be set null value, I have check the log file, it realy send
SET province = 'NY', city=null, age=null
someone tell me why? Because I want to use ajax to update one column every request...