2表副本從

Question

爲什麼沒有在甲骨文這個選擇的工作，儘管在PostgreSQL的工作（第三總和應該是在同一個這樣的：從員工選擇SUM（工資））：

select e1.employee_id, e1.department_id, e1.manager_id, e1.salary, 
sum(e2.salary), sum(e3.salary) 
from employees e1, employees e2, employees e3 
where e1.department_id=e2.department_id 
group by e1.employee_id, e1.department_id, e1.manager_id, e1.salary 
order by 1,2; 

它的工作原理在PostgreSQL中，但不在這裏。在Oracle工作只有一個副本E2沒有任何約束條件（當e1爲原始表，並通過它的主鍵分組）之後，從：

select e1.employee_id, e1.department_id, e1.manager_id, e1.salary, 
    sum(e2.salary) 
    from employees e1, employees e2 
    where e1.department_id=e2.department_id 
    group by e1.employee_id, e1.department_id, e1.manager_id, e1.salary 
    order by 1,2; 

或甚至沒有，其中作爲第一表的每個記錄中的整個總和（這裏沒有任何限制）：

select e1.employee_id, e1.department_id, e1.manager_id, e1.salary, 
    sum(e2.salary) 
    from employees e1, employees e2 
    group by e1.employee_id, e1.department_id, e1.manager_id, e1.salary 
    order by 1,2; 

當然，我知道其他替代選擇，例如：

select employee_id, department_id, manager_id, salary, 
sum(salary) over (partition by department_id) as suma, 
sum(salary) over() 
from employees order by 2,3; 

或與子查詢：

select employee_id, department_id, manager_id, salary, 
(select sum(e2.salary) from employees e2 
where e2.department_id=e1.department_id) as suma1, 
(select sum(e2.salary) from employees e2) as suma2 
from employees e1 order by 1,2; 

您是否知道在Oracle中執行多個表副本的方法？

Answer 1

在第一個查詢您創建一個完整的笛卡爾乘積，因爲你沒有設置任何約束表之間的匹配