PetaPoco returning a PIVOT query as an IEnumerable<dynamic>

Repository.cs

using System.Collections.Generic;
using PetaPoco;

namespace PetaPocoPivot.Services
{
    public class Repository : IRepository
    {
        private readonly Database _database = new Database("AdventureWorks");

        public IEnumerable<dynamic> GetEmployeeReport()
        {
            var pivotColumns = GetDepartments();

            var pivotQuery =
                string.Format(@"SELECT *
                                FROM (SELECT EmployeeID, LastName, FirstName, Department, StartDate
                                      FROM HumanResources.vEmployeeDepartmentHistory) As EmployeeData
                                PIVOT (MIN(StartDate) FOR Department IN ({0})) as EmployeePivot", pivotColumns);

            return _database.Query<dynamic>(pivotQuery);
        }

        private string GetDepartments()
        {
            var query = Sql.Builder
                .Select("DISTINCT '[' + Department + ']'")
                .From("HumanResources.vEmployeeDepartmentHistory")
                .OrderBy("1");

            return string.Join(",", _database.Query<string>(query));
        }
    }
}