I am looking to ensure that a field is unique in a table.

For example. In a user table I want to know that the UserName is unique.

So the query would normally be "Select count(UserName) as UC From Table2 Where UserName = @UserName"

then I would do something like if UC <> 0 then raise exception.

Assumptions: 1. New Record or 2. UserName has changed in the application but not yet saved to DB.

How would I create this in LLBL ? Any examples you can forward me to?

You could perform a Scalar query with count:

TableCollection coll = new TableCollection();
int numerOfRecords = (int)coll.GetScalar(TableFieldIndex.TableId, 
    null, AggregateFunction.Count, 
    (TableFlields.UserName == 'someValue'));