class OrdinalIgnoreCaseEqualityComparer: IEqualityComparer<string>
{
  public bool Equals(string x, string y)
    => x?.Equals(y, StringComparison.OrdinalIgnoreCase) ?? false;
  public int GetHashCode(string obj)
    => obj.ToLowerInvariant().GetHashCode();
}

var xd = new XmlDocument();
using(var cnn = new SqlConnection("Data Source=.;Initial Catalog=AdventureWorks;Integrated Security=True"))
{
  cnn.Open();
  using (var cmd = new SqlCommand("set showplan_xml on", cnn))
    cmd.ExecuteNonQuery();
  using(var cmd = new SqlCommand(@"
select d.Name, e.LoginID, e.JobTitle, h.StartDate
from HumanResources.Department d
join HumanResources.EmployeeDepartmentHistory h on h.DepartmentId=d.DepartmentId and h.EndDate is null
join HumanResources.Employee e on e.BusinessEntityID=h.BusinessEntityID", cnn))
  using (var reader = cmd.ExecuteReader())
  {
    reader.Read();
    xd.LoadXml(reader.GetString(0));
  }
  using (var cmd = new SqlCommand("set showplan_xml off", cnn))
    cmd.ExecuteNonQuery();
}
xd.SelectNodes("//*[@Database and @Schema and @Table]")
  ?.Cast<XmlElement>()
  .Select(e => $"{e.GetAttribute("Database")}.{e.GetAttribute("Schema")}.{e.GetAttribute("Table")}")
  .Distinct(new OrdinalIgnoreCaseEqualityComparer())
  .ToList()
  .ForEach(Console.WriteLine);

[AdventureWorks].[HumanResources].[Department]
[AdventureWorks].[HumanResources].[EmployeeDepartmentHistory]
[AdventureWorks].[HumanResources].[Employee]