在SQL中，為什么SELECT語句中的所有non-aggregated列都應包含在GROUP BY中？

我看到一條語句，上面寫著“當使用聚合函數（COUNT（），CASE表示條件邏輯）時，GROUP BY子句應該包括所有non-aggregated列，這些列都是selected.”，這是什么原因？

例如，我有一個查詢，其中這是不正確的：

select 
e.first_name, 
e.last_name, 
count(o.order_id) as num_orders, 
(case when shipped_date <= required_date then 'On Time' else 'Late' end) 
as shipped
from employees e
join orders o on e.employee_id = o.employee_id
group by e.employee_id
order by last_name, first_name, num_orders Desc

但這是正確的：

select 
e.first_name, 
e.last_name, 
count(o.order_id) as num_orders, 
(case when shipped_date <= required_date then 'On Time' else 'Late' end) 
as shipped
from employees e
join orders o on e.employee_id = o.employee_id
group by e.first_name,e.last_name,shipped
order by last_name, first_name, num_orders Desc