Hi ,
It is not recommended to touch alfresco database as Its implementation is private to Alfresco and changes from time to time. Accessing the DB directly is also likely to upset Alfresco's normal running and bypass lots of Alfresco functionality such as rules and policies.
You can only access audit tables.
The user details are stored as a Person Node within Alfresco.
use the alfresco person service. Or use the alfresco node service.