Question

我有一个使用多个SELECTS的查询，我需要对此进行优化，但是我不知道如何执行此操作。

查询：

SELECT e.last_name, e.salary, t1.PROMEDIO 
FROM employees e, 
     (
       SELECT e.department_id, AVG(e.salary) PROMEDIO 
       FROM employees e 
       GROUP  BY e.department_id
     ) t1 
WHERE e.department_id = t1.department_id 
     AND e.salary < t1.PROMEDIO;

Answer 1

如果我正确理解了您的问题，则您正在尝试退还薪水低于每个部门平均薪水的所有员工。如果是这样，您可以使用窗口函数avg() over()：

select * 
from (
    select last_name, department_id, salary, 
         avg(salary) over (partition by department_id) avgsalary
    from employees
) t
where salary < avgsalary

Online Demo

Answer 2

最好使用窗口函数编写此查询：

select e.* 
from (select last_name, department_id, salary, 
             avg(salary) over (partition by department_id) as avgsalary
      from employees e
     ) e
where salary < avgsalary;

为了提高性能，您希望在employees(department_id, salary)上建立索引。

如何通过多选优化SQL查询

2 个答案: