It is possible to select the connection to the database within the code using a dynamically created connect string... take the following code as an example for Oracle:

DATABASE demo

MAIN

DEFINE username VARCHAR(255)
DEFINE passwd VARCHAR(255)
DEFINE server VARCHAR(255)
DEFINE connectStr VARCHAR(255)

#

LET server = dbserver --this is the name of your database server

LET connectStr = username CLIPPED,"/",passwd CLIPPED, "@",server CLIPPED

## the following line switches to the username and password specified..
## .. by forming the command 'DATABASE username/password@server'

DATABASE connectStr
END MAIN


You can use a full connect string (eg. user/password@server) with the DATABASE keyword allowing you to dynamically switch database users.