Is it possible with a UserName / Password database to set a Master UserName / Password to access all records for admin purposes.
When I need to do this, I usually would use a field in the users table that stores a user access level that I can check or a combination of bits to determine what a user can and can't access. Or, You could create an admin tool that lists user accounts, so when an administrator clicks on a user name, they are automatically logged in as that user for administrative purposes. Sorry to be so vague, but your question could have a lot of different answers
For a particular database, you can set an admin username and password kind of thing using grant all privilege sql statement.
You would need to make a special login script where the user enters the user they want to be logged in as and an admin username and password