you can already check a database for a value with out breaking the code.
if you want to check if a username or email already exists in the database, create a lookup recordset that will filter the username or email address entered in the form on corresponding database column.
a dreamweaver recordset will create a variable that contains the total rows of the recordset, it will be named:
where RecordsetName is the name of the recordset.
you can add a server validation that uses this variable:
in server validations select numeric validation.
set the server variable to use the totalRows variable:
set the minimum number to -1 and the maximum number to 0.
this will cause validation to fail if the recordset is not empty