Some databases need to know the value’s type even if the value itself is NULL.
For this reason, for maximum portability,
it’s the JDBC specification itself that requires the type to be specified and MyBatis needs to pass it along since it’s build on top of JDBC.

在给?赋值的时候，如果赋的是Null，需要调用PreparedStatement#setNull()方法。

可以在Mapper层中使用@MapKey注解来指定使用什么来作为返回值Map的键。