Unique identifier for chemical substances?

One famous database for chemical substances is [PubChem](https://pubchem.ncbi.nlm.nih.gov/). If one reads the [section about other identifiers than the PubChem ID for Ethanol](https://pubchem.ncbi.nlm.nih.gov/compound/702#section=Other-Identifiers) one gets several CAS numbers (potentially deprecated ones as well), potentially several European Community (EC) Numbers and a lot of other identifiers which seem to be not really unique.


Is there a true unique identifier for chemical substances reasonable to be used in a database?


