参考：http://www.sqlite.org/lang_corefunc.html

max(X,Y,...)

The multi-argument max() function returns the argument with the maximum value,or return NULL if any argument is NULL. The multi-argument max() function searches its arguments from left to right for an argument that defines a collating function and uses that collating function for all string comparisons. If none of the arguments to max() define a collating function,then the BINARY collating function is used. Note that max() is a simple function when it has 2 or more arguments but operates as an aggregate function if given only a single argument.