我想知道我是否可以在另一个关键字中使用 SQL 关键字，例如 COUNT(DISTINCT(<columnname> ))</columnname>

Question

SELECT AVG(salary) FROM emp where COUNT(DISTINCT(dept)) > 3 ;

Using this query, i want to calculate the average salary of Departments where employee number is more than 3.

Answer 1

You can use HAVING:

SELECT AVG(salary) 
FROM emp 
HAVING COUNT(DISTINCT dept) > 3 ;

I am not sure what is it exactly you need but here is a demo of my example and one more example with a function inside a function DEMO

Above is answer for Oracle. For sqlserver I suggest to use "function inside of function" like this:

select avg(tolumn1)
from (select count(salary) as tolumn1
      from emp) A

Answer 2

i want to calculate the average salary of Departments where employee number is more than 3.

As I interpret the question, you want:

select dept, avg(salary)
from emp
group by dept
having count(*) > 3;

If you want the average across all departments, then it is not clear if you want the average of the department averages. Or the average of the employees in those departments. Either way, you need a subquery:

select avg(dept_avg) as average_of_department_salaries,
       sum(sum_salary) / sum(num_emp) as employee_average_salary
from (select dept, avg(salary) as dept_avg, count(*) as num_emp, sum(salary) as sum_salary
      from emp
      group by dept
      having count(*) > 3
     ) d

Answer 3

Select Avg(E.Salary),D.Department_name from Departments AS D inner join Employee AS E on D.Dept_Id = E.Dept_Id group by Department_name having count(Emp_Id)>3

The required result for the problem cannot be obtained without a join on Department table and the Employee table. Hence using alias for the two tables with a Group by clause followed by Having clause.