Column Aliasing

Column aliasing in dbExpression is used to eliminate ambiguity in identical column names in a returned rowset, or to simply change the name of a column in a returned rowset.
For example, the following query would return two columns with a name of Id:

IEnumerable<dynamic> purchases = db.SelectMany(
        dbo.Person.Id,
        dbo.Person.LastName,
        dbo.Purchase.Id,
        dbo.Purchase.TotalPurchaseAmount
    )
    .From(dbo.Purchase)
    .InnerJoin(dbo.Person).On(dbo.Purchase.PersonId == dbo.Person.Id)
    .Execute();

Execution of this would cause the following runtime exception during mapping to a dynamic object: DbExpression.Sql.DbExpressionException : An element with the same key 'Id' already exists in the ExpandoObject.

This is corrected by providing a field-level alias on one (or more) of the Id fields by using the As(...) method:

IEnumerable<dynamic> purchases = db.SelectMany(
        dbo.Person.Id.As("PersonId"),
        dbo.Person.LastName,
        dbo.Purchase.Id,
        dbo.Purchase.TotalPurchaseAmount
    )
    .From(dbo.Purchase)
    .InnerJoin(dbo.Person).On(dbo.Purchase.PersonId == dbo.Person.Id)
    .Execute();