如何找到Oracle SQL中员工最多的部门

Question

我正试图在演示人力资源模式中找到员工最多的部门：

SELECT d.DEPARTMENT_ID, d.DEPARTMENT_NAME, MAX(COUNT(e.EMPLOYEE_ID))
FROM departments d INNER JOIN employees e ON d.DEPARTMENT_ID = e.DEPARTMENT_ID
GROUP BY d.DEPARTMENT_ID, d.DEPARTMENT_NAME

该脚本在没有MAX()的情况下工作。

Answer 1

您可以包装当前查询（减去MAX）并选择人数最多的部门组。

WITH CTE (DEPARTMENT_ID, DEPARTMENT_NAME, empCount) AS
(
    SELECT d.DEPARTMENT_ID, d.DEPARTMENT_NAME, COUNT(e.EMPLOYEE_ID) AS empCount
    FROM departments d INNER JOIN employees e ON d.DEPARTMENT_ID = e.DEPARTMENT_ID
    GROUP BY d.DEPARTMENT_ID, d.DEPARTMENT_NAME
)
SELECT DEPARTMENT_ID, DEPARTMENT_NAME
FROM CTE
WHERE empCount = (SELECT MAX(t.empCount) FROM CTE t)

Answer 2

您可以尝试通过desc on count（e.EMPLOYEE_ID）获取带顺序的部门，并通过将结果限制在第一行来使用ROWNUM，如下所示：

select * from (select d.DEPARTMENT_ID, d.DEPARTMENT_NAME, count(e.EMPLOYEE_ID) cnt 
from departments d inner join employees e on d.DEPARTMENT_ID = e.DEPARTMENT_ID 
group by d.DEPARTMENT_ID, d.DEPARTMENT_NAME order by 3 desc) where ROWNUM=1

坦率地说，我无法针对Oracle数据库测试这个查询，因为我的机器上只安装了Derby数据库，而且这不支持ROWNUM psuedo列。

Answer 3

请查找查询：

SELECT *
FROM
(
SELECT d.DEPARTMENT_ID, d.DEPARTMENT_NAME ,COUNT(e.EMPLOYEE_ID) EMP_COUNT
FROM HR.departments d
INNER JOIN HR.employees e
ON d.DEPARTMENT_ID = e.DEPARTMENT_ID
GROUP BY d.DEPARTMENT_ID, d.DEPARTMENT_NAME
ORDER BY EMP_COUNT DESC
)
WHERE ROWNUM=1

Answer 4

使用类似的ORDER BY子句

select d.DEPARTMENT_ID, d.DEPARTMENT_NAME, 
count(e.EMPLOYEE_ID) as employe_count
from departments d
inner join employees e
on d.DEPARTMENT_ID = e.DEPARTMENT_ID
group by d.DEPARTMENT_ID, d.DEPARTMENT_NAME
order by count(e.EMPLOYEE_ID) desc
offset 0 rows fetch next 1 rows only;

注意：OFFSET和FETCH NEXT仅可从Oracle 12.1 开始使用

Answer 5

您可以使用COUNT（）OVER（）分析函数。

例如，使用HR模式中的标准employees表：

SQL> SELECT DISTINCT department_id,
  2    COUNT(department_id) OVER(PARTITION BY department_id) rn
  3  FROM hr.employees t
  4  ORDER BY rn DESC;
DEPARTMENT_ID         RN
------------- ----------
           50         45
           80         34
           30          6
          100          6
           60          5
           90          3
           20          2
          110          2
           10          1
           40          1
           70          1
                       0
12 rows selected.
SQL>

要从以上结果集中找到员工人数最多的department_id：

SQL> SELECT department_id
  2  FROM
  3    (SELECT department_id,
  4      row_number() OVER(ORDER BY rn DESC) rn
  5    FROM
  6      ( SELECT DISTINCT department_id,
  7        COUNT(department_id) OVER(PARTITION BY department_id) rn
  8      FROM hr.employees t
  9      )
 10    )
 11  WHERE rn = 1;
DEPARTMENT_ID
-------------
           50
SQL>

Answer 6

select dname,deptno,loc from dept where deptno=(select deptno from 
(select deptno,count(*) as emp_count from emp group by deptno order by emp_count desc) where rownum=1)