This function returns a pointer to the value in the environment or NULL if there is no match.
这个函数会返回一个 指向环境中的值的指针，或者如果不匹配则返回 NULL。
2
Notice also that in case of error, the function returns NULL.
另请注意，函数在出错的情况下返回 NULL。
3
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.