Once an application variable is bound to a parameter marker, the association remains until it is overridden or until the corresponding statement handle is freed.
一旦应用程序变量被绑定到某个参数标记，这种关联就会一直保持下去，直到相应的语句句柄被释放。
2
If the remote data source cannot handle the null value in a host variable or parameter marker, any function that involves binding a nullable value has to be evaluated locally.