Signature Description Parameters
template<typename T>
std::vector<T>
get_col_unique_values (const char *name) const;
It returns a vector of unique values in the named column in the same order that exists in the column.
For this method to compile and work, 3 conditions must be met:
  1. Type T must be hash-able. If this is a user defined type, you must enable and specialize std::hash.
  2. The equality operator (==) must be well defined for type T.
  3. Type T must match the actual type of the named column. Of course, if you never call this method in your application, you need not be worried about these conditions.
T: Data type of the named column