public void TestAggr()
{
Assert.Equal(4, command.Count(x => x.Name));
Assert.Equal(1, command.CountDistinct(x => x.Name));
Assert.Equal(10, command.Sum <int>(x => x.Age));
Assert.Equal(4, command.Max <int>(x => x.Age));
Assert.Equal(1, command.Min <int>(x => x.Age));
Assert.Equal(2.5, command.Avg <double>(x => x.Age));
}