sql - 如何让所有员工的工资低于平均工资？

Question

我可以获得员工人数和平均工资，但是当我尝试获得列出低于平均工资的员工人数的附加选择时，它失败了。

select count(employee_id),avg(salary) 
from employees
Where salary < avg(salary);

Answer 1

问题是这AVG是一个聚合函数。SQL 不够聪明，无法弄清楚如何在行中混合聚合结果。传统的方法是使用连接：

select count(*), avg(e.salary),
        sum(case when e.salary < const.AvgSalary then 1 else 0 end) as NumBelowAverage
from employees e cross join
     (select avg(salary) as AvgSalary from employees) as const

Answer 2

select count(*), (select avg(salary) from employees)
from employees 
where salary < (select avg(salary) from employees);

Answer 3

不清楚您希望在结果集中包含哪些列，这使得您很难回答您的问题。使问题清晰可提高答案的质量。

你似乎想要 3 个事实：

1. 在职员工人数。
2. 平均工资。
3. 收入低于平均工资的员工人数。

并且您显示了一个查询，该查询完成了前两个事实的工作：

SELECT COUNT(*) AS NumberOfEmployees,
       AVG(Salary) AS AverageSalary
  FROM Employees

COUNT(*)和有什么区别COUNT(Employee_ID)？不同之处在于后者只计算 Employee_ID 列中存在非 NULL 值的行。一个好的优化器会识别 Employee_ID 是一个主键并且不包含 NULL 值，并且查询将是相同的。但是COUNT(*)更传统，对优化器的依赖更少。

其他统计信息可以通过子查询在选择列表中生成为简单值：

SELECT COUNT(*) AS NumberOfEmployees,
       AVG(Salary) AS AverageSalary,
       (SELECT COUNT(*)
          FROM Employees
         WHERE Salary < (SELECT AVG(Salary) FROM Employees)
       ) AS NumberOfEmployeesPaidSubAverageWages
  FROM Employees

在很多情况下，这样写子查询是不合适的，但是对于指定查询的解释，就可以了。

Answer 4

   select TotalNumberOfEmployees,
          AverageSalary,
          count(e.employee_id) NumberOfEmployeesBelowAverageSalary
     from (
            select count(employee_id) TotalNumberOfEmployees,
                   avg(salary) AverageSalary
              from employees
          ) preagg
left join employees e on e.salary < preagg.AverageSalary
 group by TotalNumberOfEmployees,
          AverageSalary

Note: I used a LEFT join so if you had 3 equal employees, it would show 0 instead of no results (nobody below below average).

Answer 5

SELECT e.ename,e.deptno,e.sal,d.avg
FROM emp e,(SELECT deptno, avg(sal) avg
             FROM emp
             GROUP BY deptno) d
 WHERE e.deptno=d.deptno
 AND
 e.sal < d.avg

Answer 6

select * from <table name> where salary < (select avg(<salary column name) from <table name>);

例子：

select * from EMPLOYEE where sal < (select avg(emp_sal) from EMPLOYEE);